Process for flashing a reaction medium comprising lithium acetate

ABSTRACT

A process for producing acetic acid is disclosed, in which the methyl iodide concentration is maintained in the vapor product stream formed in a flashing step. The process includes introducing a lithium compound into the reactor and maintaining a concentration of lithium acetate in the reaction medium is in an amount from 0.3 to 0.7 wt. %. The reaction medium is fed forward to the flashing step. The methyl iodide concentration in the vapor product stream ranges from 24 to less than 36 wt. %, based on the weight of the vapor product stream. The vapor product stream is distilled in a first column to obtain an acetic acid product stream comprising acetic acid and hydrogen iodide in an amount of less than or equal to 300 wppm and/or methyl iodide in an amount from 0.1 to 6 wt. %.

CROSS-REFERENCES TO RELATED APPLICATIONS

This patent application is a continuation-in-part and claims priorityfrom U.S. patent application Ser. No. 14/789,006, entitled “ProcessesFor Flashing A Reaction Medium,” filed Jul. 1, 2015, and claims priorityfrom U.S. Provisional Application No. 62/080,035, entitled “ProcessesFor Producing Acetic Acid by Introducing a Lithium Compound,” filed Nov.14, 2014, the disclosure of which are incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

This invention relates to processes for producing acetic acid and, inparticular, to improved processes for flashing a crude acetic acidproduct in an acetic acid production system.

BACKGROUND OF THE INVENTION

Among currently employed processes for synthesizing acetic acid, one ofthe most useful, commercially, is the catalyzed carbonylation ofmethanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329,which is incorporated herein by reference in its entirety. Thecarbonylation catalyst contains a metal catalyst, such as rhodium, whichis either dissolved or otherwise dispersed in a liquid reaction mediumor supported on an inert solid, along with a halogen-containing catalystpromoter as exemplified by methyl iodide. The reaction is conducted bycontinuously bubbling carbon monoxide gas through a liquid reactionmedium in which the catalyst is dissolved.

Methanol and carbon monoxide are fed to a reactor as feedstocks. Aportion of the reaction medium is continuously withdrawn and provided toa flash vessel where the product is flashed and sent as a vapor to apurification train. The purification train includes a light ends columnwhich removes “light” or low boiling components as an overhead andprovides a sidedraw stream for further purification. The purificationtrain may further include columns to dehydrate the sidedraw stream orcolumns for removing “heavy” or high boiling components, such aspropionic acid, from the sidedraw stream. It is desirable in acarbonylation process for making acetic acid to minimize the number ofdistillation operations to minimize energy usage in the process.

U.S. Pat. No. 5,416,237 discloses a process for the production of aceticacid by carbonylation of methanol in the presence of a rhodiumcarbonylation catalyst, methyl iodide and an iodide salt stabilizer bymaintaining a finite concentration of water of up to about 10% by weightand a methyl acetate concentration of at least 2% by weight in theliquid reaction medium and recovering the acetic acid product by passingthe liquid reaction medium through a flash zone to produce a vaporfraction which is passed to a single distillation column from which anacetic acid product is removed. The vapor fraction comprises water up toabout 8% by weight, acetic acid product, propionic acid by-product andthe majority of the methyl acetate and methyl iodide.

U.S. Pat. No. 7,820,855 discloses a carbonylation process for producingacetic acid including: (a) carbonylating methanol or its reactivederivatives in the presence of a Group VIII metal catalyst and methyliodide promoter to produce a liquid reaction mixture including aceticacid, water, methyl acetate and methyl iodide; (b) feeding the liquidreaction mixture at a feed temperature to a flash vessel which ismaintained at a reduced pressure; (c) heating the flash vessel whileconcurrently flashing the reaction mixture to produce a crude productvapor stream. The selection of the reaction mixture and the flow rate ofthe reaction mixture fed to the flash vessel as well as the amount ofheat supplied to the flash vessel are controlled, such that thetemperature of the crude product vapor stream is maintained at atemperature less than 90° F. cooler than the feed temperature of theliquid reaction mixture to the flash vessel, and the concentration ofacetic acid in the crude product vapor stream is greater than 70% byweight of the crude product vapor stream. Through the flash vessel theproduct acetic acid and the majority of the light ends (methyl iodide,methyl acetate, and water) are separated from the reactor catalystsolution, and the crude process stream is forwarded with dissolved gasesto the distillation or purification section in single stage flash. Themethyl iodide concentrations decrease as the temperature of the flashvessel increases and the flow rates decrease.

U.S. Pat. No. 9,006,483 discloses a production process of acetic acidthat seeks to inhibit the concentration of hydrogen iodide and provide aliquid-liquid separation of an overhead from a distillation column.Acetic acid is produced by distilling a mixture containing hydrogeniodide, water, acetic acid and methyl acetate in a first distillationcolumn to form an overhead and a side cut stream or bottom streamcontaining acetic acid, cooling and condensing the overhead in acondenser to form separated upper and lower phases in a decanter.According to this process, a zone having a high water concentration isformed in the distillation column above the feed position of the mixtureby feeding a mixture having a water concentration of not less than aneffective amount to not more than 5% by weight (e.g., 0.5 to 4.5% byweight) and a methyl acetate concentration of 0.5 to 9% by weight (e.g.,0.5 to 8% by weight) as the mixture to the distillation column anddistilling the mixture. In the zone having a high water concentration,hydrogen iodide is allowed to react with methyl acetate to producemethyl iodide and acetic acid.

The need remains for improved acetic acid production processes havingimproved separation steps, increased production capacities and loweroperating costs.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is directed to a process forproducing acetic acid comprising carbonylating a reactant feed stream inthe presence of water, rhodium catalyst, iodide salt and methyl iodideto form a reaction medium in a reactor. The reactant feed streamcomprises methanol, methyl acetate, dimethyl ether, or mixtures thereof.The process further comprises introducing a lithium compound into thereactor, maintaining a concentration of lithium acetate in the reactionmedium in an amount from 0.3 to 0.7 wt. %, separating the reactionmedium in a flash vessel to form a liquid recycle stream and a vaporproduct stream, which comprises acetic acid in an amount from 45 to 75wt. %, methyl iodide in an amount from 24 to less than 36 wt. %, methylacetate in an amount of less than or equal to 9 wt. %, water in anamount of less than or equal to 14 wt. %. The process further comprisesdistilling at least a portion of the vapor product stream in a firstcolumn to obtain an acetic acid product stream comprising acetic acidand hydrogen iodide in an amount of less than or equal to 300 wppm,e.g., less than or equal to 50 wppm, and an overhead stream comprisingmethyl iodide, water and methyl acetate. In some embodiments, the vaporproduct stream may comprise acetaldehyde in an amount from 0.005 to 1wt. %, e.g., preferably from 0.01 to 0.8 wt. % and more preferably from0.01 to 0.7 wt. %. In a further embodiment, the vapor product stream maycomprises acetic acid in an amount from 55 to 75 wt. %, methyl iodide inan amount from 24 to 35 wt. %, methyl acetate in an amount from 0.5 to 8wt. %, water in an amount from 0.5 to 14 wt. %, and hydrogen iodide inan amount less than or equal to 0.5 wt. %. In another embodiment, thevapor product stream comprises acetic acid in an amount from 60 to 70wt. %, methyl iodide in an amount from 25 to 35 wt. %, methyl acetate inan amount from 0.5 to 6.5 wt. %, water in an amount from 1 to 8 wt. %,and hydrogen iodide in an amount less than or equal to 0.1 wt. %. Insome embodiments, the process further comprises venting a gaseous streamfrom the reactor that comprises hydrogen iodide in an amount of lessthan 1 wt. %, e.g., from 0.001 to 1 wt. %.

In one embodiment, the overhead stream is phase separated to form alight liquid phase and a heavy liquid phase. The light liquid phase maycomprise acetic acid in an amount from 1 to 40 wt. %, methyl iodide inan amount of less than or equal to 10 wt. %, methyl acetate in an amountfrom 1 to 50 wt. %, water in an amount from 40 to 80 wt. %, acetaldehydein an amount of less than or equal to 5 wt. %, and hydrogen iodide in anamount of less than or equal to 1 wt. %. In a preferred embodiment, thelight liquid phase comprises acetic acid in an amount from 5 to 15 wt.%, methyl iodide in an amount of less than or equal to 3 wt. %, methylacetate in an amount from 1 to 15 wt. %, water in an amount from 70 to75 wt. %, acetaldehyde in an amount from 0.1 to 0.7 wt. %, and hydrogeniodide in an amount from 0.001 to 0.5 wt. %. In a further embodiment, aportion of the heavy liquid phase may be treated to remove at least onepermanganate reducing compound selected from the group consisting ofacetaldehyde, acetone, methyl ethyl ketone, butylaldehyde,crotonaldehyde, 2-ethyl crotonaldehyde, 2-ethyl butyraldehyde, and thealdol condensation products thereof.

In one embodiment, the liquid recycle stream comprises a metal catalystin an amount from 0.01 to 0.5 wt. %, lithium iodide in an amount from 5to 20 wt. %, corrosion metals in an amount from 10 to 2500 wppm, aceticacid in an amount from 60 to 90 wt. %, methyl iodide in an amount from0.5 to 5 wt. %, methyl acetate in an amount from 0.1 to 5 wt. %, waterin an amount from 0.1 to 8 wt. %, acetaldehyde in an amount from 0.0001to 1 wt. %, and hydrogen iodide in an amount from 0.0001 to 0.5 wt. %.

In one embodiment, the lithium compound is selected from the groupconsisting of lithium acetates, lithium carboxylates, lithiumcarbonates, lithium hydroxides, and mixtures thereof. The concentrationof methyl acetate in the reaction medium may be greater than theconcentration of the lithium acetate. The process may further comprisesmaintaining a concentration of hydrogen iodide in the reaction mediumfrom 0.1 to 1.3 wt. %. The reaction may be conducted while maintaining acarbon monoxide partial pressure from 2 to 30 atm and a hydrogen partialpressure in the reactor that is less than or equal to 0.04 atm.

In one embodiment, the acetic acid product stream may comprise methylacetate in an amount from 0.1 to 6 wt. %, hydrogen iodide in an amountof less than or equal to 300 wppm, e.g., less than or equal to 50 wppm,the water concentration is maintained in the acetic acid product streamfrom 1 to 9 wt. %, and/or methyl iodide in an amount from 0.1 to 6 wt.%. the acetic acid product stream comprises each of the methyl iodideand the methyl acetate in an amount within the range of ±0.9 wt. % ofthe water concentration in the side stream. The acetic acid productstream may comprise each of the methyl iodide and the methyl acetate inan amount within the range of ±0.9 wt. % of the water concentration inthe side stream.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be better understood in view of the appendednon-limiting FIGURE, wherein:

FIG. 1 is a schematic drawing for producing acetic acid in accordancewith the present invention.

DETAILED DESCRIPTION OF THE INVENTION

At the outset, it should be noted that in the development of any suchactual embodiment, numerous implementation—specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem related and business related constraints, which will vary fromone implementation to another. In addition, the processes disclosedherein can also comprise components other than those cited orspecifically referred to, as is apparent to one having average orreasonable skill in the art.

In the summary and this detailed description, each numerical valueshould be read once as modified by the term “about” (unless alreadyexpressly so modified), and then read again as not so modified unlessotherwise indicated in context. Also, in the summary and this detaileddescription, it should be understood that a concentration range listedor described as being useful, suitable, or the like, is intended thatany and every concentration within the range, including the end points,is to be considered as having been stated. For example, a range “from 1to 10” is to be read as indicating each and every possible number alongthe continuum between about 1 and about 10. Thus, even if specific datapoints within the range, or even no data points within the range, areexplicitly identified or refer to only a few specific data points, it isto be understood that inventors appreciate and understand that any andall data points within the range are to be considered to have beenspecified, and that inventors possessed knowledge of the entire rangeand all points within the range.

Throughout the entire specification, including the claims, the followingterms have the indicated meanings unless otherwise specified.

As used in the specification and claims, “near” is inclusive of “at.”The term “and/or” refers to both the inclusive “and” case and theexclusive “or” case, and is used herein for brevity. For example, amixture comprising acetic acid and/or methyl acetate may comprise aceticacid alone, methyl acetate alone, or both acetic acid and methylacetate.

All percentages are expressed as weight percent (wt. %), based on thetotal weight of the particular stream or composition present, unlessotherwise noted. Room temperature is 25° C. and atmospheric pressure is101.325 kPa unless otherwise noted.

For purposes herein:

acetic acid may be abbreviated as “AcOH”;

acetaldehyde may be abbreviated as “AcH”;

methyl acetate may be abbreviated “MeAc”;

methanol may be abbreviated “MeOH”;

methyl iodide may be abbreviated as “MeI”;

hydrogen iodide may be abbreviated as “HI”;

carbon monoxide may be abbreviated “CO”; and

dimethyl ether may be abbreviated “DME”.

HI refers to either molecular hydrogen iodide or dissociated hydriodicacid when at least partially ionized in a polar medium, typically amedium comprising at least some water. Unless otherwise specified, thetwo are referred to interchangeably. Unless otherwise specified, HIconcentration is determined via acid-base titration using apotentiometric end point. In particular, HI concentration is determinedvia titration with a standard lithium acetate solution to apotentiometric end point. It is to be understood that for purposesherein, the concentration of HI is not determined by subtracting aconcentration of iodide assumed to be associated with a measurement ofcorrosion metals or other non H+ cations from the total ionic iodidepresent in a sample.

It is to be understood that HI concentration does not refer to iodideion concentration. HI concentration specifically refers to HIconcentration as determined via potentiometric titration.

This subtraction method is an unreliable and imprecise method todetermine relatively lower HI concentrations (i.e., less than about 5weight percent) due to the fact that it assumes all non-H+ cations (suchas cations of Fe, Ni, Cr, Mo) are associated with iodide anionexclusively. In reality, a significant portion of the metal cations inthis process can be associated with acetate anion. Additionally, many ofthese metal cations have multiple valence states, which adds even moreunreliability to the assumption on the amount of iodide anion whichcould be associated with these metals. Ultimately, this method givesrise to an unreliable determination of the actual HI concentration,especially in view of the ability to perform a simple titration directlyrepresentative of the HI concentration.

For purposes herein, an “overhead” or “distillate” of a distillationcolumn refers to at least one of the lower boiling condensable fractionswhich exits at or near the top, (e.g., proximate to the top), of thedistillation column, and/or the condensed form of that stream orcomposition. Obviously, all fractions are ultimately condensable, yetfor purposes herein, a condensable fraction is condensable under theconditions present in the process as readily understood by one of skillin the art. Examples of noncondensable fractions may include nitrogen,hydrogen, and the like. Likewise, an overhead stream may be taken justbelow the upper most exit of a distillation column, for example, whereinthe lowest boiling fraction is a non-condensable stream or represents ade-minimis stream, as would be readily understood by one of reasonableskill in the art.

The “bottoms” or “residuum” of a distillation column refers to one ormore of the highest boiling fractions which exit at or near the bottomof the distillation column, also referred to herein as flowing from thebottom sump of the column. It is to be understood that a residuum may betaken from just above the very bottom exit of a distillation column, forexample, wherein the very bottom fraction produced by the column is asalt, an unusable tar, a solid waste product, or a de-minimis stream aswould be readily understood by one of reasonable skill in the art.

For purposes herein, distillation columns comprise a distillation zoneand a bottom sump zone. The distillation zone includes everything abovethe bottom sump zone, i.e., between the bottom sump zone and the top ofthe column. For purposes herein, the bottom sump zone refers to thelower portion of the distillation column in which a liquid reservoir ofthe higher boiling components is present (e.g., the bottom of adistillation column) from which the bottom or residuum stream flows uponexiting the column. The bottom sump zone may include reboilers, controlequipment, and the like.

It is to be understood that the term “passages”, “flow paths”, “flowconduits”, and the like in relation to internal components of adistillation column are used interchangeably to refer to holes, tubes,channels, slits, drains, and the like, which are disposed through and/orwhich provide a path for liquid and/or vapor to move from one side ofthe internal component to the other side of the internal component.Examples of passages disposed through a structure, such as a liquiddistributor of a distillation column, include drain holes, drain tubes,drain slits, and the like, which allow a liquid to flow through thestructure from one side to another. Average residence time is defined asthe sum total of all liquid volume hold-up for a given phase within adistillation zone divided by the average flow rate of that phase throughthe distillation zone. The hold-up volume for a given phase can includeliquid volume contained in the various internal components of the columnincluding collectors, distributors and the like, as well as liquidcontained on trays, within downcomers, and/or within structured orrandom packed bed sections.

Vapor Product Stream

The production of acetic acid via the carbonylation of methanol involvesthe formation of a reaction medium in a reactor, and flashing thereaction medium in a flash vessel to form a liquid recycle stream and avapor product stream. The vapor product stream is then distilled in oneor more distillation columns to remove byproducts and form an aceticacid product. The present invention provides processes for producingacetic acid while reducing byproduct formation by maintaining a specificmethyl iodide concentration in the vapor product stream formed in theflash step. Methyl iodide is a useful promoter for the carbonylationcatalyst. During separation, however, methyl iodide tends to concentratewith the acetic acid that is separated from the reaction medium.Consequently, to avoid costly losses through fugitive emissions and toreduce iodide impurities in the acetic acid product, the methyl iodidemust be separated from the acetic acid and returned to the reactionmedium.

According to the present invention, methyl iodide concentrations aremaintained at a sufficient level in the vapor product to supportincreased production rates while reducing the amount of methyl iodidethat must be recovered from the vapor product stream. In one embodiment,the process may also comprise introducing a lithium compound into thereactor and maintaining a concentration of lithium acetate in thereaction medium in an amount from 0.3 to 0.7 wt. %. As described herein,it is advantageous to reduce methyl iodide in the reactor by maintainingthese levels of lithium acetate. Reducing the methyl iodide in thereactor by using lithium acetate may advantageously control the methyliodide in the vapor product stream in an amount from 24 to less than 36wt. %.

Reducing the amount of methyl iodide advantageously debottlenecks thedistillation columns due to the lower amounts of methyl iodide that mustbe separated. Debottlenecking the distillation columns advantageouslyincreases production capacities and lowers operating costs. Maintainingmethyl iodide at desired levels in the vapor product also beneficiallykeeps the hydrogen iodide concentrations in the downstream distillationcolumns at low levels and thus minimizes corrosion to the columns.

In one embodiment, the present invention is directed to a process forproducing acetic acid comprising carbonylating a reactant feed stream inthe presence of water, a rhodium catalyst, an iodide salt and a methyliodide to form a reaction medium in a reactor. Said reactant feed streamcomprises methanol, methyl acetate, dimethyl ether, or mixtures thereof.The process further comprises introducing a lithium compound into thereactor, maintaining a concentration of lithium acetate in the reactionmedium in an amount from 0.3 to 0.7 wt. %, separating the reactionmedium in a flash vessel to form a liquid recycle stream and a vaporproduct stream, which comprises acetic acid in an amount from 45 to 75wt. %, methyl iodide in an amount from 24 to less than 36 wt. %, methylacetate in an amount of less than or equal to 9 wt. %, water in anamount of less than or equal to 14 wt. %, and distilling at least aportion of the vapor product stream in a first column to obtain anacetic acid product stream comprising acetic acid and hydrogen iodide inan amount of less than or equal to 300 wppm and an overhead streamcomprising methyl iodide, water and methyl acetate.

As shown in U.S. Pat. No. 7,820,855, production rates decrease as methyliodide concentrations decrease, based on mass flow rate from the reactorto the flash vessel. The present invention can maintain higherproduction rates without incurring the decrease in mass flow rate asdescribed in U.S. Pat. No. 7,820,855. Methyl iodide at a concentrationless than 24 wt. % results in undesirably low production rates. On theother hand, vapor product streams having methyl iodide at concentrationsgreater than or equal to 36 wt. % increase the load on the distillationcolumns needed to remove methyl iodide, which adversely impactsproduction capacity. Methyl iodide cannot be eliminated in the vaporproduct stream using the flash vessel and thus must be recovered throughthe distillation columns. Maintaining methyl iodide concentration in therange from 24 to less than 36 wt. % in the vapor product is important tocontrol hydrogen iodide formation, which is resulted from the hydrolysisof methyl iodide. Hydrogen iodide is a known corrosion-causing compoundand may undesirably concentrate with methyl iodide concentrations above36 wt. %. in the vapor product stream. Thus, having a vapor productstream with 24 wt. % to less than 36 wt. % methyl iodide may provide thedesired hydrogen iodide control.

The vapor product may be sampled using on-line measuring techniques tomeasure methyl iodide content and provide real-time or near real-timefeedback. Sampling the vapor product stream on-line is easier thansampling the liquid reaction medium. In addition, the concentration ofthe methyl iodide in the vapor product is related to and provides anindirect indication of the concentration of methyl iodide in thereactor. The ability to maintain a consistent methyl iodideconcentration in the vapor product stream is useful to set up a schedulefor adding methyl iodide to the reactor. For example, in a commercialprocess, small amounts of methyl iodide are lost due to fugitiveemissions and the use of various purge streams in the separation system.As the methyl iodide concentration in the vapor product decreases,additional methyl iodide may be added to the reactor. Conversely, whenthe methyl iodide concentration is too high, a portion of the heavyliquid phase from the light ends column may be purged from the system.

In addition to methyl iodide, the vapor product stream also comprisesacetic acid, methyl acetate, and water. By-products such as hydrogeniodide, acetaldehyde, and propionic acid may also be present in thevapor product stream. The reactants, i.e., methanol and carbon monoxide,when not consumed, may be recovered in the vapor product stream. In oneembodiment, the vapor product stream comprises acetic acid in an amountfrom 45 to 75 wt. %, methyl iodide in an amount from 24 to less than 36wt. %, methyl acetate in an amount of less than or equal to 9 wt. %, andwater in an amount of less than or equal to 14 wt. %, based on the totalweight of the vapor product stream. More preferably, the vapor productstream comprises acetic acid in an amount from 55 to 75 wt. %, methyliodide in an amount from 24 to 35 wt. %, methyl acetate in an amountfrom 0.5 to 8 wt. %, water in an amount from 0.5 to 14 wt. %,acetaldehyde in an amount from 0.01 to 0.8 wt. %, and hydrogen iodide inan amount less than or equal to 0.5 wt. %. In yet a further preferredembodiment, the vapor product stream acetic acid in an amount from 60 to70 wt. %, methyl iodide in an amount from 25 to 35 wt. %, methyl acetatein an amount from 0.5 to 6.5 wt. %, water in an amount from 1 to 8 wt.%, acetaldehyde in an amount from 0.01 to 0.7 wt. %, and hydrogen iodidein an amount less than or equal to 0.1 wt. %.

The acetaldehyde concentration in the vapor product stream may be from0.005 to 1 wt. %, e.g., from 0.01 to 0.8 wt. %, or from 0.01 to 0.7 wt.%, based on the total weight of the vapor product stream. Inembodiments, the acetaldehyde may be present in amounts of less than orequal to 1 wt. %, e.g., less than or equal to 0.9 wt. %, less than orequal to 0.8 wt. %, less than or equal to 0.7 wt. %, less than or equalto 0.6 wt. %, or less than or equal to 0.5 wt. %, and/or theacetaldehyde may be present in amounts of greater than or equal to 0.005wt. %, e.g., greater than or equal to 0.01 wt. %, greater than or equalto 0.05 wt. %, or greater than or equal to 0.1 wt. %. In addition toacetaldehyde, there may also be other permanganate reducing compounds(“PRC's”), such as acetone, methyl ethyl ketone, butylaldehyde,crotonaldehyde, 2-ethyl crotonaldehyde, 2-ethyl butyraldehyde, and thealdol condensation products thereof. In one embodiment, a suitablepotassium permanganate test is JIS K1351 (2007). These compounds, ifpresent in the vapor product stream, are generally in an amount similarto or less than the acetaldehyde concentrations. In one embodiment, isdesirable to remove the PRC's, including acetaldehyde, to maintain lowconcentrations of PRC's in the vapor product stream. This may reduce theimpurity/byproduct formation in the reactor.

The vapor product stream may comprise hydrogen iodide in an amount ofless than or equal to 1 wt. %, based on the total weight of the vaporproduct stream, e.g., less than or equal to 0.5 wt. %, or less than orequal to 0.1 wt. %. In terms of ranges, hydrogen iodide may be presentin amounts from 0.0001 to 1 wt. %, e.g., from 0.0001 to 0.5 wt. %, from0.0001 to 0.1 wt. %. In some embodiments, when hydrogen iodide iscontrolled in the reactor, hydrogen iodide may be present in an amountof less than 0.0001 wt. %. These lower amounts are usually slightlyabove the detection limits.

The vapor product stream is preferably substantially free of, i.e.,contains less than 0.0001 wt. %, propionic acid, based on the totalweight of the vapor product stream.

In one embodiment, there is provided a process for producing aceticacid, comprising separating a reaction medium in a flash vessel to forma liquid recycle stream and a vapor product stream, which comprisesacetic acid in an amount from 45 to 75 wt. %, methyl iodide in an amountfrom 24 to less than 36 wt. %, methyl acetate in an amount of less thanor equal to 9 wt. %, water in an amount of less than or equal to 14 wt.%, acetaldehyde in an amount from 0.005 to 1 wt. %, and hydrogen iodidein an amount less than or equal to 1 wt. %. The process furthercomprises distilling at least a portion of the vapor product stream in afirst column to obtain an acetic acid product stream comprising aceticacid and hydrogen iodide in an amount of less than or equal to 300 wppmand an overhead stream comprising methyl iodide, water and methylacetate.

The present invention also advantageously facilitates maintaining awater balance in the separation system and, in particular, during thedistillation steps by controlling the net production of water. As aresult, the invention beneficially inhibits or prevents increases inwater content that may necessitate the purging of water from the system.Purging of water can also adversely result in the loss of catalystpromoters such as methyl iodide. In exemplary embodiments, the netproduction of water in the distilling step increases by less than orequal to 0.5%, e.g., by less than or equal to 0.1% or by less than orequal to 0.05%, over the water concentration in the vapor product streamthat is fed to the distilling step. In contrast, U.S. Pat. No. 9,006,483describes promoting reactions that lead to the formation of water andallows for adding more water to the distilling step. Increases in thenet production of water would be expected to be higher due to thepromotion of these reactions and additions, leading to increased load ondistillation equipment.

The vapor product stream is fed to a distillation column, e.g., a firstcolumn, which may also be referred to as a light ends column. In oneoptional embodiment, a portion of the vapor product stream may becondensed. The first column separates the vapor product stream to forman overhead stream, a product stream, and optionally a bottoms stream.The acetic acid product stream may be withdrawn as a sidedraw stream,and more preferably as a liquid sidedraw stream. In one embodiment, theacetic acid product stream primarily comprises acetic acid and may alsocomprise water, methyl iodide, methyl acetate, or hydrogen iodide. Theacetic acid product stream withdrawn in the sidedraw preferablycomprises acetic acid in an amount of greater than or equal to 90 wt. %acetic acid, based on the total weight of the sidedraw stream, e.g.,greater than or equal to 94 wt. % or greater than or equal to 96 wt. %.In terms of ranges, the acetic acid product stream comprises acetic acidin an amount from 90 to 99.5 wt. %, e.g., 90 to 99 wt. %, or from 91 to98 wt. %. Such concentrations allow a majority of the acetic acid fed tothe first column to be withdrawn in the sidedraw stream for furtherpurification. Although minor amounts of acetic acid may be present,acetic acid is preferably not recovered as a product in the overhead orbottoms of the first column.

The process preferably includes a step of maintaining a waterconcentration in the sidedraw stream in an amount from 1 to 9 wt. %,e.g., from 1 to 3 wt. %, and more preferably from 1.1 to 2.5 wt. %. Inembodiments, the concentration of water in the side stream is maintainedat greater than or equal to 1 wt. %, or greater than or equal to 1.1 wt.%, or greater than or equal to 1.3 wt. %, or greater than or equal to1.5 wt. %, or greater than or equal to 2 wt. %, and/or in embodiments,the concentration of water in the side stream is maintained at less thanor equal to 3 wt. %, or less than or equal to 2.8 wt. %, or less than orequal to 2.5 wt. %, or less than or equal to 2.1 wt. %. In embodiments,the concentration of hydrogen iodide in the side stream is maintained atless than or equal to 300 wppm, e.g., less than or equal to 275 wppm,less than or equal to 250 wppm, less than or equal to 225 wppm, lessthan or equal to 175 wppm, or less than or equal to 50 wppm, and/or inembodiments, the concentration of hydrogen iodide in the side stream ismaintained at greater than or equal to 0.05 wppm, e.g., greater than orequal to 0.1 wppm, greater than or equal to 1 wppm, greater than orequal to 5 wppm, greater than or equal to 10 wppm or greater than orequal to 50 wppm. In terms of ranges, the sidedraw stream preferablycomprises hydrogen iodide in an amount from 0.05 to 300 wppm, based onthe total weight of the sidedraw stream, e.g., from 0.1 to 50 wppm, orfrom 5 to 30 wppm. Hydrogen iodide is soluble in acetic acid-watermixtures containing water in an amount from 3 to 8 wt. %, and thesolubility of hydrogen iodide decreases as the water concentrationdecreases. This correlation results in increasing hydrogen iodidevolatility, which leads to reduced amounts of hydrogen iodide beingcollected in the overhead of the column. Although hydrogen iodide hasbeen indicated by others to be corrosive, a certain amount of hydrogeniodide under some conditions may beneficially act as a catalyst, such asa catalyst for forming dimethyl ether as described in U.S. Pat. No.7,223,883 (describing the benefits of dimethyl ether formation incertain acetic acid separation processes), the entirety of which isincorporated herein by reference.

In addition to acetic acid and water, the sidedraw stream may alsocomprise one or more C₁-C₁₄ alkyl iodides in an amount from 0.1 to 6 wt.%, e.g., from 0.5 to 5 wt. %, from 0.6 to 4 wt. %, from 0.7 to 3.7 wt.%, or from 0.8 to 3.6 wt. %. In one embodiment, the one or more C₁-C₁₄alkyl iodides comprise methyl iodide. Other alkyl iodides such as hexyliodide may also be formed from carbonyl impurities such as acetaldehyde.More preferably, the sidedraw stream comprises one or more C₁-C₁₄ alkyliodides in an amount from 0.5 to 3 wt. %. Due to the presence of water,the sidedraw stream may also contain methyl acetate in an amount from0.1 to 6 wt. %, e.g., from 0.5 to 5 wt. %, from 0.6 to 4 wt. %, from 0.7to 3.7 wt. %, or from 0.8 to 3.6 wt. %.

In one embodiment, a process is provided for producing acetic acidcomprising separating a reaction medium in a flash vessel to form aliquid recycle stream and a vapor product stream comprising acetic acidin an amount from 45 to 75 wt. %, methyl iodide in an amount from 24 toless than 36 wt. %, methyl acetate in an amount of less than or equal to9 wt. %, water in an amount of less than or equal to 14 wt. %,acetaldehyde in an amount from 0.005 to 1 wt. %, and hydrogen iodide inan amount less than or equal to 1 wt. %. The process further comprisesdistilling at least a portion of the vapor product stream in a firstcolumn to obtain an acetic acid product stream comprising acetic acidand methyl iodide in an amount from 0.1 to 6 wt. % and an overheadstream comprising methyl iodide, water and methyl acetate. The processpreferably also includes a step of maintaining a water concentration inthe sidedraw stream in an amount from 1 to 9 wt. %, e.g., from 1 to 3wt. %. In one embodiment, the concentration of hydrogen iodide in theside stream is maintained at less than or equal to 300 wppm.

In one embodiment, a process is provided for producing acetic acidcomprising separating a reaction medium in a flash vessel to form aliquid recycle stream and a vapor product stream comprising acetic acidin an amount from 45 to 75 wt. %, methyl iodide in an amount from 24 toless than 36 wt. %, methyl acetate in an amount of less than or equal to9 wt. %, water in an amount of less than or equal to 14 wt. %,acetaldehyde in an amount from 0.005 to 1 wt. %, and hydrogen iodide inan amount less than or equal to 1 wt. %. The process further comprisesdistilling at least a portion of the vapor product stream in a firstcolumn to obtain an acetic acid product stream comprising acetic acid,and methyl acetate in an amount from 0.1 to 6 wt. %, and obtain anoverhead stream comprising methyl iodide, water and methyl acetate. Theprocess preferably also includes a step of maintaining a waterconcentration in the sidedraw stream in an amount from 1 to 9 wt. %,e.g., from 1 to 3 wt. %. In one embodiment, the concentration ofhydrogen iodide in the side stream is maintained at less than or equalto 300 wppm.

In one embodiment, a process is provided for producing acetic acidcomprising separating a reaction medium in a flash vessel to form aliquid recycle stream and a vapor product stream comprising acetic acidin an amount from 45 to 75 wt. %, methyl iodide in an amount from 24 toless than 36 wt. %, methyl acetate in an amount of less than or equal to9 wt. %, water in an amount of less than or equal to 14 wt. %,acetaldehyde in an amount from 0.005 to 1 wt. %, and hydrogen iodide inan amount less than or equal to 1 wt. %. The process further comprisesdistilling at least a portion of the vapor product stream in a firstcolumn to obtain an acetic acid product stream comprising acetic acid,methyl iodide in an amount from 0.1 to 6 wt. %, and methyl acetate in anamount from 0.1 to 6 wt. %, and obtain an overhead stream comprisingmethyl iodide, water and methyl acetate. The process preferably alsoincludes a step of maintaining a water concentration in the sidedrawstream in an amount from 1 to 9 wt. %, e.g., from 1 to 3 wt. %. In oneembodiment, the concentration of hydrogen iodide in the side stream ismaintained at less than or equal to 300 wppm.

As provided herein, in embodiments, there may be a stable amount ofother reactor components and impurities, such as C₁-C₁₄ alkyl iodides,namely methyl iodide, and methyl acetate in the sidedraw stream based onthe water concentration. By stable amount it is meant that theconcentration of the one or more C₁-C₁₄ alkyl iodides and theconcentration of methyl acetate is within the range of ±0.9 wt. % of thewater concentration in the side stream, e.g., ±0.7 wt. %, ±0.6 wt. %,±0.5 wt. %, ±0.4 wt. %, ±0.3 wt. %, ±0.2 wt. %, or ±0.1 wt. %. Forexample, when the water concentration is 2.5 wt. %, the concentration ofC₁-C₁₄ alkyl iodides is from 1.6 to 3.4 wt. %, and the concentration ofmethyl acetate is from 1.6 to 3.4 wt. %. This may be achieved bycontrolling a recycle rate of a portion of the light liquid phase to thereactor. In some embodiments, controlling the recycle rate of a portionof the light liquid phase to the reactor may achieve a stableconcentration of methyl iodide in the side stream of within the range of±0.6 wt. % of the water concentration in the side stream, e.g., ±0.5 wt.%, ±0.4 wt. %, ±0.3 wt. %, ±0.2 wt. %, or ±0.1 wt. %.

In one embodiment, a process is provided for producing acetic acidcomprising carbonylating a reactant feed stream in the presence ofwater, rhodium catalyst, iodide salt and methyl iodide to form areaction medium in a reactor. The reactant feed stream comprisesmethanol, methyl acetate, dimethyl ether, or mixtures thereof. Theprocess further comprises introducing a lithium compound into thereactor, maintaining a concentration of lithium acetate in the reactionmedium in an amount from 0.3 to 0.7 wt. %, separating the reactionmedium in a flash vessel to form a liquid recycle stream and a vaporproduct stream comprising acetic acid in an amount from 45 to 75 wt. %,methyl iodide in an amount from 24 to less than 36 wt. %, methyl acetatein an amount of less than or equal to 9 wt. %, water in an amount ofless than or equal to 14 wt. The process further comprises distilling atleast a portion of the vapor product stream in a first column to obtainan acetic acid product stream comprising acetic acid and hydrogen iodidein an amount of less than or equal to 300 wppm and obtain an overheadstream comprising methyl iodide, water and methyl acetate. The aceticacid product stream comprises each of the methyl iodide and the methylacetate in an amount ±0.9 wt. % of the water concentration in the sidestream. In one embodiment, the vapor stream may also compriseacetaldehyde in an amount from 0.005 to 1 wt. %.

Maintaining the desired methyl iodide concentration in the vapor productstream is an improvement over other processes that focus on water andmethyl acetate concentrations in the vapor product that feeds the firstcolumn, such as those described in U.S. Pat. No. 9,006,483. Unlike waterand methyl acetate, methyl iodide has significant value and the loss ofwhich can represent a substantial loss in cost. One of the advantages ofthe process disclosed herein is that it separates methyl iodide from therest and returns it to the reactor, which allows for the recovery ofthis valuable catalyst promoter.

Hydrogen iodide concentration of the sidedraw stream is determined bypotentiometric titration using lithium acetate as the titrant. Othershave determined hydrogen iodide content indirectly by calculation. USPub. No. 2013/0310603, for example, indicates that iodide ionconcentration may be calculated by subtracting the iodide ionconcentration derived from the iodide salt form (including iodidesderived from co-catalysts and metal iodide) from the total concentrationof iodide ion (I⁻). Such indirect calculation techniques are typicallyinaccurate, resulting in a poor indication of actual hydrogen iodideconcentration owing largely to the inaccuracies of the underlying ionmeasurement methods. In addition, this indirect calculation techniquefails to account for other iodide forms because metal cations aremeasured and incorrectly assumed to be completely associated only withiodide anions while, in fact, the metal cations may be associated withother anions, such as acetate and catalyst anions. In contrast, thedirect measurement of hydrogen iodide concentration according to thepresent invention advantageously reflects the actual hydrogen iodideconcentration in the system, and can result in accuracy as low as 0.01%.In one embodiment, the hydrogen iodide concentration in the side streammay be determined by potentiometric titration using lithium acetate asthe titrant.

Reaction Step

An exemplary reaction and acetic acid recovery system 100 is shown inFIG. 1. As shown, methanol-containing feed stream 101 and carbonmonoxide-containing feed stream 102 are directed to liquid phasecarbonylation reactor 105, in which the carbonylation reaction occurs toform acetic acid.

Methanol-containing feed stream 101 may comprise at least one memberselected from the group consisting of methanol, dimethyl ether, andmethyl acetate. Methanol-containing feed stream 101 may be derived inpart from a fresh feed or may be recycled from the system. At least someof the methanol and/or reactive derivative thereof may be converted tomethyl acetate in the liquid reaction medium by esterification withacetic acid.

Typical reaction temperatures for carbonylation may be from 150 to 250°C., with the range of 180 to 225° C. being preferred. The carbonmonoxide partial pressure in the reactor may vary widely but istypically from 2 to 30 atm, e.g., from 3 to 10 atm. The hydrogen partialpressure in the reactor is typically from 0.05 to 2 atm, e.g., from 0.25to 1.9 atm. In some embodiments, the present invention may be operatedwith a hydrogen partial pressure from 0.3 to 2 atm, e.g., from 0.3 to1.5 atm, or from 0.4 to 1.5 atm. Because of the partial pressure ofby-products and the vapor pressure of the contained liquids, the totalreactor pressure may range from 15 to 40 atm. The production rate ofacetic acid may be from 5 to 50 mol/L·h, e.g., from 10 to 40 mol/L·h,and preferably from 15 to 35 mol/L·h.

Carbonylation reactor 105 is preferably either a mechanically-stirredvessel, a vessel with an educted or pump-around mixing, or bubble-columntype vessel, with or without an agitator, within which the reactingliquid or slurry contents are maintained, preferably automatically, apredetermined level, which preferably remains substantially constantduring normal operation. Into carbonylation reactor 105, fresh methanol,carbon monoxide, and sufficient water are continuously introduced asneeded to maintain suitable concentrations in the reaction medium.

The metal catalyst may comprise a Group VIII metal. Suitable Group VIIIcatalysts include rhodium and/or iridium catalysts. When a rhodiumcatalyst is used, the rhodium catalyst may be added in any suitable formsuch that rhodium is in the catalyst solution as an equilibrium mixtureincluding [Rh(CO)₂I₂]-anion, as is well known in the art. Iodide saltsoptionally maintained in the reaction mixtures of the processesdescribed herein may be in the form of a soluble salt of an alkali metalor alkaline earth metal, quaternary ammonium, phosphonium salt ormixtures thereof. In certain embodiments, the catalyst co-promoter islithium iodide, lithium acetate, or mixtures thereof. The catalystco-promoter may be added as a non-iodide salt that will generate aniodide salt. The catalyst co-promoter may be introduced directly intothe reaction system. Alternatively, the iodide salt may be generatedin-situ since under the operating conditions of the reaction system, awide range of non-iodide salt precursors will react with methyl iodideor hydroiodic acid in the reaction medium to generate the correspondingcatalyst co-promoter. For additional detail regarding rhodium catalysisand iodide salt generation, see U.S. Pat. Nos. 5,001,259; 5,026,908;5,144,068 and 7,005,541, which are incorporated herein by reference intheir entireties. The carbonylation of methanol utilizing iridiumcatalyst is well known and is generally described in U.S. Pat. Nos.5,942,460, 5,932,764, 5,883,295, 5,877,348, 5,877,347 and 5,696,284,which are incorporated herein by reference in their entireties.

The halogen-containing catalyst promoter of the catalyst system mayinclude an organic halide, such as an alkyl, aryl, and substituted alkylor aryl halides. Preferably, the halogen-containing catalyst promoter ispresent in the form of an alkyl halide. Even more preferably, thehalogen-containing catalyst promoter is present in the form of an alkylhalide in which the alkyl radical corresponds to the alkyl radical ofthe feed alcohol, which is being carbonylated. Thus, in thecarbonylation of methanol to acetic acid, the halide promoter willinclude methyl halide, and more preferably methyl iodide. In oneembodiment, the methyl iodide concentration is maintained in the vaporproduct stream at a concentration from 24 wt. % to less than 36 wt. %.In one embodiment, the reaction medium may have methyl iodideconcentration of 7 wt. % or less, e.g., from 4 to 7 wt. %.

The components of the reaction medium are maintained within definedlimits to ensure sufficient production of acetic acid. The reactionmedium contains a concentration of the metal catalyst, e.g., rhodiumcatalyst, in an amount from 200 to 3000 wppm, e.g., from 800 to 3000wppm, or from 900 to 1500 wppm. The concentration of water in thereaction medium is maintained to be less than or equal to 14 wt. %,e.g., from 0.1 wt. % to 14 wt. %, from 0.2 wt. % to 10 wt. % or from0.25 wt. % to 5 wt. %. Preferably, the reaction is conducted under lowwater conditions and the reaction medium contains water in an amountfrom 0.1 to 4.1 wt. %, e.g., from 0.1 to 3.1 wt. % or from 0.5 to 2.8wt. %. The concentration of methyl iodide in the reaction medium ismaintained to be from 3 to 20 wt. %, e.g., from 4 to 13.9 wt. %, or from4 to 7 wt. %. The concentration of iodide salt, e.g., lithium iodide, inthe reaction medium is maintained to be from 1 to 25 wt. %, e.g., from 2to 20 wt. %, from 3 to 20 wt. %. The concentration of methyl acetate inthe reaction medium is maintained to be from 0.5 to 30 wt. %, e.g., from0.3 to 20 wt. %, from 0.6 to 9 wt. %, or from 0.6 to 4.1 wt. %. Theseamounts are based on the total weight of the reaction medium. Theconcentration of acetic acid in the reaction medium is generally greaterthan or equal to 30 wt. %, e.g., greater than or equal to 40 wt. % orgreater than or equal to 50 wt. %.

Lithium Acetate in Reaction Medium

In embodiments, the process for producing acetic acid further includesintroducing a lithium compound into the reactor to maintain theconcentration of lithium acetate in an amount from 0.3 to 0.7 wt. % inthe reaction medium. Without being bound by theory lithium acetate inthe reaction medium in these concentrations may reduce the methyl iodidein the reaction medium and thus allow for controlling the methyl iodidein the vapor stream to be less than 36 wt. % as described herein. Alsothe introduction of the lithium compound into the reactor helps tostabilize the rhodium catalyst and thus reduces the amount of methyliodide in the reaction medium required for achieving the suitableactivity. Without introducing the lithium compound, additional rhodiumwould be needed when the methyl iodide concentration in the reactionmedium decreases.

In embodiments, an amount of the lithium compound is introduced into thereactor to maintain the concentration of hydrogen iodide in an amountfrom 0.1 to 1.3 wt. % in the reaction medium. In embodiments, theconcentration of the rhodium catalyst is maintained in an amount from200 to 3000 wppm in the reaction medium, the concentration of water ismaintained in amount from 0.1 to 4.1 wt. % in the reaction medium, andthe concentration of methyl acetate is maintained from 0.6 to 4.1 wt. %in the reaction medium, based on the total weight of the reaction mediumpresent in the carbonylation reactor.

In embodiments, the lithium compound introduced into the reactor isselected from the group consisting of lithium acetate, lithiumcarboxylates, lithium carbonates, lithium hydroxide, other organiclithium salts, and mixtures thereof. In embodiments, the lithiumcompound is soluble in the reaction medium. In an embodiment, lithiumacetate dihydrate may be used as the source of the lithium compound.

Lithium acetate reacts with hydrogen iodide according to the followingequilibrium reaction (I) to form lithium iodide and acetic acid:LiOAc+HI⇄LiI+HOAc  (I)

Lithium acetate is thought to provide improved control of hydrogeniodide concentration relative to other acetates, such as methyl acetate,present in the reaction medium. Without being bound by theory, lithiumacetate is a conjugate base of acetic acid and thus reactive towardhydrogen iodide via an acid-base reaction. This property is thought toresult in an equilibrium of the reaction (I) which favors reactionproducts over and above that produced by the corresponding equilibriumof methyl acetate and hydrogen iodide. This improved equilibrium isfavored by water concentrations of less than or equal to 4.1 wt. % inthe reaction medium. In addition, the relatively low volatility oflithium acetate compared to methyl acetate allows the lithium acetate toremain in the reaction medium except for volatility losses and smallamounts of entrainment into the vapor crude product. In contrast, therelatively high volatility of methyl acetate allows the material todistill into the purification train, rendering methyl acetate moredifficult to control. Lithium acetate is much easier to maintain andcontrol in the process at consistent low concentrations of hydrogeniodide. Accordingly, a relatively small amount of lithium acetate may beemployed relative to the amount of methyl acetate needed to controlhydrogen iodide concentrations in the reaction medium. It has furtherbeen discovered that lithium acetate is at least three times moreeffective than methyl acetate in promoting methyl iodide oxidativeaddition to the rhodium [I] complex.

In embodiments, the concentration of lithium acetate in the reactionmedium is maintained at greater than or equal to 0.3 wt. %, or greaterthan or equal to 0.35 wt. %, or greater than or equal to 0.4 wt. %, orgreater than or equal to 0.45 wt. %, or greater than or equal to 0.5 wt.%, and/or in embodiments, the concentration of lithium acetate in thereaction medium is maintained at less than or equal to 0.7 wt. %, orless than or equal to 0.65 wt. %, or less than or equal to 0.6 wt. %, orless than or equal to 0.55 wt. %.

In one embodiment, there is provided a process for producing aceticacid, comprising carbonylating a reactant feed stream comprisingmethanol, methyl acetate, dimethyl ether, or mixtures thereof in areactor in the presence of water, rhodium catalyst, iodide salt andmethyl iodide to form a reaction medium in a reactor, introducing alithium compound into the reactor, maintaining a concentration oflithium acetate in the reaction medium in an amount from 0.3 to 0.7 wt.%, separating the reaction medium in a flash vessel to form a liquidrecycle stream and a vapor product stream comprising acetic acid in anamount from 45 to 75 wt. %, methyl iodide in an amount from 24 to lessthan 36 wt. %, methyl acetate in an amount of less than or equal to 9wt. %, water in an amount of less than or equal to 14 wt. %. The processfurther comprises distilling at least a portion of the vapor productstream in a first column to obtain an acetic acid product streamcomprising acetic acid and hydrogen iodide in an amount of less than orequal to 300 wppm and obtain an overhead stream comprising methyliodide, water and methyl acetate.

It has been discovered that an excess of lithium acetate in the reactionmedium can adversely affect the other compounds in the reaction medium,leading to decrease productivity. Conversely, it has been discoveredthat a lithium acetate concentration in the reaction medium below 0.3wt. % is unable to maintain the desired hydrogen iodide concentrationsin the reaction medium of below 1.3 wt. %.

In embodiments, the lithium compound may be introduced continuously orintermittently into the reaction medium. In embodiments, the lithiumcompound is introduced during reactor start up. In embodiments, thelithium compound is introduced intermittently to replace entrainmentlosses.

In some embodiments, the desired reaction rates are obtained even at lowwater concentrations by maintaining an ester concentration in thereaction medium of the desired carboxylic acid and an alcohol, desirablythe alcohol used in the carbonylation, and an additional iodide ion thatis over and above the iodide ion that is present as hydrogen iodide. Adesired ester is methyl acetate. The additional iodide ion is desirablyan iodide salt, with lithium iodide being preferred. It has been foundthat under low water concentrations, methyl acetate and lithium iodideact as rate promoters only when relatively high concentrations of eachof these components are present and that the promotion is higher whenboth of these components are present simultaneously.

Carbonylation Reaction

The carbonylation reaction of methanol to acetic acid product may becarried out by contacting the methanol feed with gaseous carbonmonoxide, bubbled through an acetic acid solvent reaction mediumcontaining the rhodium catalyst, methyl iodide promoter, methyl acetate,and additional soluble iodide salt, at conditions of temperature andpressure suitable to form the carbonylation product. It will begenerally recognized that it is the concentration of iodide ion in thecatalyst system that is important and not the cation associated with theiodide, and that at a given molar concentration of iodide, the nature ofthe cation is not as significant as the effect of the iodideconcentration. Any metal iodide salt, or any iodide salt of any organiccation, or other cations such as those based on amine or phosphinecompounds (optionally, ternary or quaternary cations), can be maintainedin the reaction medium provided that the salt is sufficiently soluble inthe reaction medium to provide the desired level of the iodide. When theiodide is a metal salt, it is preferred that it is an iodide salt of amember of the group consisting of the metals of Group IA and Group IIAof the periodic table, as set forth in the “Handbook of Chemistry andPhysics” published by CRC Press, Cleveland, Ohio, 2002-03 (83rdedition). In particular, alkali metal iodides are useful, with lithiumiodide being particularly suitable. In the low water carbonylationprocess, the additional iodide ion over and above the iodide ion presentas hydrogen iodide is generally present in the catalyst solution inamounts such that the total iodide ion concentration is from 1 to 25 wt.% and the methyl acetate is generally present in amounts from 0.5 to 30wt. %, and the methyl iodide is generally present in amounts from 1 to25 wt. %. The rhodium catalyst is generally present in amounts from 200to 3000 wppm.

The reaction medium may also contain impurities that should becontrolled to avoid byproduct formation. These impurities tend toconcentrate in the vapor stream. One impurity in the reaction medium maybe ethyl iodide, which is difficult to separate from acetic acid.Applicant has further discovered that the formation of ethyl iodide maybe affected by numerous variables, including the concentration ofacetaldehyde, ethyl acetate, methyl acetate and methyl iodide in thereaction medium. Additionally, ethanol content in the methanol source,hydrogen partial pressure and hydrogen content in the carbon monoxidesource have been discovered to affect ethyl iodide concentration in thereaction medium and, consequently, propionic acid concentration in thefinal acetic acid product.

In embodiments, the propionic acid concentration in the acetic acidproduct may further be maintained below 250 wppm by maintaining theethyl iodide concentration in the reaction medium at less than or equalto 750 wppm without removing propionic acid from the acetic acidproduct.

In embodiments, the ethyl iodide concentration in the reaction mediumand propionic acid in the acetic acid product may be present in a weightratio from 3:1 to 1:2. In embodiments, the acetaldehyde:ethyl iodideconcentration in the reaction medium is maintained at a weight ratiofrom 2:1 to 20:1.

In embodiments, the ethyl iodide concentration in the reaction mediummay be maintained by controlling at least one of the hydrogen partialpressure, the methyl acetate concentration, the methyl iodideconcentration, and/or the acetaldehyde concentration in the reactionmedium.

In embodiments, the concentration of ethyl iodide in the reaction mediumis maintained/controlled to be less than or equal to 750 wppm, or e.g.,less than or equal to 650 wppm, or less than or equal to 550 wppm, orless than or equal to 450 wppm, or less than or equal to 350 wppm. Inembodiments, the concentration of ethyl iodide in the reaction medium ismaintained/controlled at greater than or equal to 1 wppm, or e.g., 5wppm, or 10 wppm, or 20 wppm, or 25 wppm, and less than or equal to 650wppm, or e.g., 550 wppm, or 450 wppm, or 350 wppm.

In embodiments, the weight ratio of ethyl iodide in the reaction mediumto propionic acid in the acetic acid product may range from 3:1 to 1:2,or e.g., from 5:2 to 1:2, or from 2:1 to 1:2, or from 3:2 to 1:2.

In embodiments, the weight ratio of acetaldehyde to ethyl iodide in thereaction medium may range from 20:1 to 2:1, or e.g., from 15:1 to 2:1 orfrom 9:1 to 2:1.

In a typical carbonylation process, carbon monoxide is continuouslyintroduced into the carbonylation reactor, desirably below the agitator,which may be used to stir the contents. The gaseous feed preferably isthoroughly dispersed through the reacting liquid by this stirring means.The temperature of the reactor may be controlled and the carbon monoxidefeed is introduced at a rate sufficient to maintain the desired totalreactor pressure. Stream 113 comprising the liquid reaction medium exitsreactor 105.

Gaseous purge stream 106 desirably is vented from the reactor 105 toprevent buildup of gaseous by-products and to maintain a set carbonmonoxide partial pressure at a given total reactor pressure. In oneembodiment, the gaseous stream 106 contains low amounts of hydrogeniodide of less than or equal to 1 wt. %, e.g., less than or equal to 0.9wt. %, less than or equal to 0.8 wt. %, less than or equal to 0.7 wt. %,less than or equal to 0.5 wt. %. Hydrogen iodide in excess of theseamounts may increase the duty on the scrubber to prevent hydrogen iodidefrom being purged. In one embodiment, using maintaining a concentrationof lithium acetate in the reaction medium in an amount from 0.3 to 0.7wt. % may also advantageously control the hydrogen iodide concentrationin the reaction medium in an amount from 0.1 to 1.3 wt. %. Lowering thehydrogen iodide in the reaction medium may also advantageously lower thehydrogen iodide in the gaseous stream. The embodiments using lithiumacetate in the reaction medium advantageously reduces hydrogen iodideand results in less hydrogen iodide being withdrawn to the flash vesselas well as less hydrogen in the gaseous stream.

This further embodiment may also comprise scrubbing the gaseous streamto remove hydrogen iodide from a purge stream. Typically the treatmentsystem is a scrubber, stripper or absorber, such as a pressure-swingabsorber.

In one embodiment a process is provided for producing acetic acid,comprising carbonylating a reactant feed stream comprising methanol,methyl acetate, dimethyl ether, or mixtures thereof in a reactor. Thereactant feed stream form a reaction medium with water, rhodiumcatalyst, iodide salt and methyl iodide in the reactor. The processfurther comprises introducing a lithium compound into the reactor,maintaining a concentration of lithium acetate in the reaction medium inan amount from 0.3 to 0.7 wt. %, venting a gaseous stream from thereactor that comprises hydrogen iodide in an amount of less than 1 wt.%. The process further comprises separating the reaction medium in aflash vessel to form a liquid recycle stream and a vapor product stream,which comprises acetic acid in an amount from 45 to 75 wt. %, methyliodide in an amount from 24 to less than 36 wt. %, methyl acetate in anamount of less than or equal to 9 wt. %, water in an amount of less thanor equal to 14 wt. %. The process further comprises distilling at leasta portion of the vapor product stream in a first column to obtain anacetic acid product stream comprising acetic acid and hydrogen iodide inan amount of less than or equal to 300 wppm and obtain an overheadstream comprising methyl iodide, water and methyl acetate.

The acetic acid production system preferably includes separation system108, employed to recover the acetic acid and recycle metal catalyst,methyl iodide, methyl acetate, and other system components within theprocess. One or more of the recycle streams may be combined prior tobeing introduced into the reaction system, which comprises the reactorand flash vessel. The separation system also preferably controls waterand acetic acid content in the carbonylation reactor, as well asthroughout the system, and facilitates permanganate reducing compound(“PRC”) removal. PRC's may include acetaldehyde, acetone, methyl ethylketone, butylaldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, 2-ethylbutyraldehyde, and the aldol condensation products thereof.

The reaction medium is drawn off from the carbonylation reactor 105 at arate sufficient to maintain a constant level therein and is provided toflash vessel 110 via stream 113. Reactor 105 and flash vessel 110, alongwith the associated pumps, vents, pipes, and values, comprise thereaction system. The flash separation may be carried out at atemperature from 80° C. to 280° C., under an absolute pressure from 0.25to 10 atm, and more preferably from 100° C. to 260° C. and from 0.3 to10 atm. In one embodiment, the flash vessel may operate under a reducedpressure relative to the reactor. In flash vessel 110, the reactionmedium is separated in a flash separation step to obtain a vapor productstream 112 comprising acetic acid and methyl iodide, as describedherein, and liquid recycle stream 111 comprising a catalyst-containingsolution. The catalyst-containing solution may be predominantly aceticacid containing the rhodium and the iodide salt along with lesserquantities of methyl acetate, methyl iodide, and water and is recycledto the reactor, as discussed above. Prior to returning liquid recycle tothe reactor, a slip stream may pass through a corrosion metal removalbed, such as an ion exchange bed, to remove any entrained corrosionmetals, such as nickel, iron, chromium, and molybdenum, as described inU.S. Pat. No. 5,731,252, which is incorporated herein by reference intheir entirety. Also, the corrosion metal removal bed may be used toremove nitrogen compounds, such as amines, as described in U.S. Pat. No.8,697,908, which is incorporated herein by reference in their entirety.

The vapor product stream 112 is described above as comprising aceticacid in an amount from 45 to 75 wt. %, methyl iodide in an amount from24 to less than 36 wt. %, methyl acetate in an amount of less than orequal to 9 wt. %, and water in an amount of less than or equal to 14 wt.%, based on the total weight of the vapor product stream. Theacetaldehyde concentration in the vapor product stream may be in anamount from 0.005 to 1 wt. %, based on the total weight of the vaporproduct stream, e.g., from 0.01 to 0.8 wt. %, or from 0.01 to 0.7 wt. %.Vapor product stream 112 may comprise hydrogen iodide in an amount lessthan or equal to 1 wt. %, based on the total weight of the vapor productstream, e.g., less than or equal to 0.5 wt. %, or less than or equal to0.1 wt. %.

Liquid recycle stream 111 comprises acetic acid, the metal catalyst,corrosion metals, as well as other various compounds. In one embodiment,liquid recycle stream comprises acetic acid in an amount from 60 to 90wt. %, metal catalyst in an amount from 0.01 to 0.5 wt. %; corrosionmetals (e.g., nickel, iron and chromium) in a total amount from 10 to2500 wppm; lithium iodide in an amount from 5 to 20 wt. %; methyl iodidein an amount from 0.5 to 5 wt. %; methyl acetate in an amount from 0.1to 5 wt. %; water in an amount from 0.1 to 8 wt. %; acetaldehyde in anamount of less than or equal to 1 wt. % (e.g., from 0.0001 to 1 wt. %acetaldehyde); and hydrogen iodide in an amount of less than or equal to0.5 wt. % (e.g., from 0.0001 to 0.5 wt. % hydrogen iodide).

In one embodiment a process is provided for producing acetic acidcarbonylating a reactant feed stream comprising methanol, methylacetate, dimethyl ether, or mixtures thereof in a reactor. The reactantfeed stream forms a reaction medium in the presence of water, a rhodiumcatalyst, an iodide salt and a methyl iodide. The process furthercomprises separating the reaction medium in a flash vessel to form aliquid recycle stream. The liquid recycle stream comprises rhodiumcatalyst in an amount from 0.01 to 0.5 wt. %, lithium iodide in anamount from 5 to 20 wt. %, corrosion metals in an amount from 10 to 2500wppm, acetic acid in an amount from 60 to 90 wt. %, methyl iodide in anamount from 0.5 to 5 wt. %, methyl acetate in an amount from 0.1 to 5wt. %, water in an amount from 0.1 to 8 wt. %. The liquid recycle streamalso comprises a vapor product stream comprising acetic acid in anamount from 45 to 75 wt. %, methyl iodide in an amount from 24 to lessthan 36 wt. %, methyl acetate in an amount of less than or equal to 9wt. % methyl acetate, water in an amount of less than or equal to 15 wt.%, and hydrogen iodide in an amount of less than or equal to 1 wt. %,and distilling at least a portion of the vapor product stream in a firstcolumn to obtain an acetic acid product stream comprising acetic acidand hydrogen iodide in an amount of less than or equal to 300 wppm andobtain an overhead stream comprising methyl iodide, water and methylacetate.

The respective flow rates of vapor product stream 112 and liquid recyclestream 111 may vary, and in one exemplary embodiment from 15% to 55% ofthe flow into flash vessel 110 is removed as vapor product stream 112,and from 45% to 85% of the flow is removed as liquid recycle stream 111.The catalyst-containing solution may be predominantly acetic acidcontaining the metal catalyst, e.g., rhodium and/or iridium, and theiodide salt along with lesser quantities of methyl acetate, methyliodide, and water and is recycled to reactor 105, as discussed above.Prior to returning the liquid recycle stream to the reactor, a slipstream may pass through a corrosion metal removal bed, such as an ionexchange bed, to remove any entrained corrosion metals as described inU.S. Pat. No. 5,731,252, which is incorporated herein by reference inits entirety. Also, the corrosion metal removal bed may be used toremove nitrogen compounds, such as amines, as described in U.S. Pat. No.8,697,908, which is incorporated herein by reference in its entirety.

In addition to acetic acid, methyl iodide and acetaldehyde, vaporproduct stream 112 also may comprise methyl acetate, water, hydrogeniodide, and other PRC's, e.g., crotonaldehyde. Dissolved gases exitingreactor 105 and entering flash vessel 110 comprise a portion of thecarbon monoxide and may also contain gaseous by-products such asmethane, hydrogen, and carbon dioxide. Such dissolved gases exit flashvessel 110 as part of the vapor product stream 112. In one embodiment,carbon monoxide in gaseous purge stream 106 may be fed to the base offlash vessel 110 to enhance rhodium stability.

Recovery of Acetic Acid

The distillation and recovery of acetic acid is not particularly limitedfor the purposes of the present invention. In one embodiment a processis provided for producing acetic acid, comprising separating a reactionmedium in a flash vessel to form a liquid recycle stream and a vaporproduct stream comprising acetic acid in an amount from 45 to 75 wt. %,methyl iodide in an amount from 24 to less than 36 wt. %, methyl acetatein an amount of less than or equal to 9 wt. %, water in an amount ofless than or equal to 14 wt. %, acetaldehyde in an amount from 0.005 to1 wt. %, and hydrogen iodide in an amount less than or equal to 1 wt. %;distilling at least a portion of the vapor product stream in a firstcolumn to obtain an acetic acid product stream comprising acetic acidand hydrogen iodide in an amount of less than or equal to 300 wppm andan overhead stream comprising methyl iodide, water and methyl acetate;condensing the low boiling overhead vapor stream and biphasicallyseparating the condensed stream to form a heavy liquid phase and a lightliquid phase, and distilling the acetic acid product stream in a secondcolumn to obtain an acetic acid product.

First Column

As shown in FIG. 1, vapor product stream 112 comprising from 24 to lessthan 36 wt. % methyl iodide is directed to a first column 120, alsoreferred to as a light ends column. In one embodiment, vapor productstream 112 comprises acetic acid, methyl acetate, water, methyl iodide,and acetaldehyde, along with other impurities such as hydrogen iodideand/or crotonaldehyde, and/or byproducts such as propionic acid.Distillation yields a low boiling overhead vapor stream 122, a purifiedacetic acid product that preferably is removed via a sidedraw stream123, and a high boiling residue stream 121. A majority of the aceticacid is removed in sidedraw stream 123 and preferably little or noacetic acid is recovered from high boiling residue stream 121. Althoughthe concentration of acetic acid may be relatively high in boilingresidue stream 121, the mass flow of the boiling residue stream 121relative to side stream 123 is very small. In embodiments, the mass flowof the boiling residue stream 121 is less than or equal to 0.75% of sidestream 123, e.g., less than or equal to 0.55%, or less than or equal to0.45%.

In one embodiment, low boiling overhead vapor stream 122 comprises waterin amount greater than or equal to 5 wt. %, e.g., greater than or equalto 10 wt. %, or greater than or equal to 25 wt. %. In terms of ranges,the low boiling overhead vapor stream 112 may comprise water in anamount from 5 wt. % to 80 wt. %, e.g., from 10 wt. % to 70 wt. % or from25 wt. % to 60 wt. %. Reducing the water concentration to less than 5wt. % is generally not advantageous because it would result in a largeacetic acid recycle stream back to the reaction system and increase therecycle stream throughout the entire purification system. In addition towater, low-boiling overhead vapor stream 122 may also comprise methylacetate, methyl iodide, and carbonyl impurities, such as PRC's, whichare preferably concentrated in the overhead vapor stream to be removedfrom acetic acid in sidedraw stream 123.

As shown, low-boiling overhead vapor stream 122 preferably is condensedand directed to an overhead phase separation unit, as shown by overheaddecanter 124. Conditions are desirably maintained such that thecondensed low-boiling overhead vapor stream 122, once in decanter 124,may separate and form a light liquid phase 133 and a heavy liquid phase134. The phase separation should maintain two separate phases, withoutforming a third phase or emulsion between the phases. An offgascomponent may be vented via line 132 from decanter 124. In embodiments,the average residence time of the condensed low-boiling overhead vaporstream 122 in overhead decanter 124 is greater than or equal to 1minute, e.g., greater than or equal to 3 minutes, greater than or equalto 5 minutes, greater than or equal to 10 minutes, and/or the averageresidence time is less than or equal to 60 minutes, e.g., less than orequal to 45 minutes, or less than or equal to 30 minutes, or less thanor equal to 25 minutes.

Although the specific compositions of light liquid phase 133 may varywidely, some exemplary compositions are provided below in Table 1.

TABLE 1 Exemplary Light Liquid Phase from Light Ends Overhead conc. (Wt.%) conc. (Wt. %) conc. (Wt. %) Water 40-80 50-75 70-75 Methyl Acetate 1-50  1-25  1-15 Acetic Acid  1-40  1-25  5-15 PRC's (AcH) <5 <3 <1Methyl Iodide <10 <5 <3 Hydrogen Iodide <1 <0.5 0.001-0.5 

In one embodiment, overhead decanter 124 is arranged and constructed tomaintain a low interface level to prevent an excess hold up of methyliodide. Although the specific compositions of heavy liquid phase 134 mayvary widely, some exemplary compositions are provided below in Table 2.

TABLE 2 Exemplary Heavy Liquid Phase from Light Ends Overhead conc. (Wt.%) conc. (Wt. %) conc. (Wt. %) Water <3 0.05-1   0.01-1   Methyl Acetate0.1-25  0.5-20  0.7-15  Acetic Acid 0.1-10  0.5-10  0.7-10  PRC's (AcH)<5 <3 0.05-0.5  Methyl Iodide 60-98 60-95 80-90 Hydrogen Iodide <1 <0.50.001-0.5 

The density of the heavy liquid phase 134 may be from 1.3 to 2, e.g.,from 1.5 to 1.8, from 1.5 to 1.75 or from 1.55 to 1.7. As described inU.S. Pat. No. 6,677,480, the measured density in the heavy liquid phase134 may correlate to the methyl acetate concentration in the reactionmedium. As density decreases, the methyl acetate concentration in thereaction medium increases. In one embodiment of the present invention,heavy liquid phase 134 is recycled to the reactor and the light liquidphase 133 is controlled to be recycled through the same pump. It may bedesirable to recycle a portion of the light liquid phase 133 that doesnot disrupt the pump and to maintain a density of the combined lightliquid phase 133 and heavy liquid phase of greater than or equal to 1.3,e.g., greater than or equal to 1.4, greater than or equal to 1.5, orgreater than or equal to 1.7. As described herein, a portion of theheavy liquid phase 134 may be treated to remove impurities, such asacetaldehyde.

As indicated by Tables 1 and 2, the water concentration in the lightliquid phase 133 is larger than the heavy liquid phase 134, and thus thepresent invention can control the side stream water concentrationthrough the recycle of the light liquid phase. The concentration ofcomponents in sidedraw stream 123, such as water and/or hydrogen iodide,may be controlled by the recycle rate of light liquid phase 133 to thereaction system. The reflux ratio (the mass flow rate of the refluxdivided by the total mass flow exiting the top of the column 120,including both heavy liquid phase 134, which may or may not be fullyrecycled, and light liquid phase 133) to the first column of the lightliquid phase 133 via line 135 preferably is from 0.05 to 0.4, e.g., from0.1 to 0.35 or from 0.15 to 0.3. In one embodiment, to reduce the refluxratio, the number of theoretical trays above the sidedraw stream and topof first column may be greater than 5, e.g., preferably greater than 10.In one embodiment, to reduce the reflux ratio, the number of theoreticaltrays above the side stream and top of first column may be greater thanor equal to 5, e.g., preferably greater than or equal to 10. In oneembodiment, a flow valve and/or flow monitor (not shown) may be used tocontrol the reflux in line 135 and recycle in line 136.

In one embodiment, the recycle of light liquid phase in line 136 back toreactor 105 is up to or equal to 20%, e.g., up to or equal to 10%, ofthe total light liquid phase 133 condensed from the column overhead(reflux plus recycle). In terms of ranges the recycle of light liquidphase in line 136 may be from 0 to 20%, e.g., from 0.1 to 20%, from 0.5to 20%, from 1 to 15%, or from 1 to 10%, of the total light liquid phase133, which is condensed from the low-boiling overhead vapor stream(reflux plus recycle). The remaining portion may be used as a reflux onthe light ends column or fed to an PRC removal system. For example,recycle in line 136 may be combined with liquid recycle stream 111 andbe returned to reactor 105. In one embodiment, recycle in line 136 maybe combined with another stream that is being recycled to the reactionsystem, e.g., reactor 105 or flash vessel 110. When condensed overheadstream 138 from drying column 125 is phase-separated to form an aqueousphase and an organic phase, the recycle in line 136 may be preferablycombined with the aqueous phase. Alternatively, recycle in line 136 maybe combined, or at least partially combined, with heavy liquid phase 134and/or the organic phase from the overhead stream 138.

PRC Removal System

Although not shown, a portion of light liquid phase 133 and/or heavyliquid phase 134 may be separated and directed to acetaldehyde or PRCremoval system to recover methyl iodide and methyl acetate during theacetaldehyde removal process. As shown in Tables 1 and 2, light liquidphase 133 and/or heavy liquid phase 134 each contain PRC's and theprocess may include removing carbonyl impurities, such as acetaldehyde,that deteriorate the quality of the acetic acid product and may beremoved in suitable impurity removal columns and absorbers as describedin U.S. Pat. Nos. 6,143,930; 6,339,171; 7,223,883; 7,223,886; 7,855,306;7,884,237; 8,889,904; and US Pub. Nos. 2006/0011462, which areincorporated herein by reference in their entirety. Carbonyl impurities,such as acetaldehyde, may react with iodide catalyst promoters to formalkyl iodides, e.g., ethyl iodide, propyl iodide, butyl iodide, pentyliodide, hexyl iodide, etc. Also, because many impurities originate withacetaldehyde, it is desirable to remove carbonyl impurities from theliquid light phase.

The portion of light liquid phase 133 and/or heavy liquid phase 134 fedto the acetaldehyde or PRC removal system may vary from 1% to 99% of themass flow of either the light liquid phase 133 and/or heavy liquid phase134, e.g., from 1 to 50%, from 2 to 45%, from 5 to 40%, 5 to 30% or 5 to20%. Also in some embodiments, a portion of both the light liquid phase133 and heavy liquid phase 134 may be fed to the acetaldehyde or PRCremoval system. The portion of the light liquid phase 133 not fed to theacetaldehyde or PRC removal system may be refluxed to the first columnor recycled to the reactor, as described herein. The portion of theheavy liquid phase 134 not fed to the acetaldehyde or PRC removal systemmay be recycled to the reactor. Although a portion of heavy liquid phase134 may be refluxed to the first column, it is more desirable to returnthe methyl iodide enriched heavy liquid phase 134 to the reactor.

In one embodiment, a portion of light liquid phase 133 and/or heavyliquid phase 134 is fed to a distillation column which enriches theoverhead thereof to have acetaldehyde and methyl iodide. Depending onthe configuration, there may be two separate distillation columns, andthe overhead of the second column may be enriched in acetaldehyde andmethyl iodide. Dimethyl ether, which may be formed in-situ, may also bepresent in the overhead. The overhead may be subject to one or moreextraction stages to remove a raffinate enriched in methyl iodide and anextractant. A portion of the raffinate may be returned to thedistillation column, first column, overhead decanter and/or reactor. Forexample, when the heavy liquid phase 134 is treated in the PRC removalsystem, it may be desirable to return a portion the raffinate to eitherthe distillation column or reactor. Also, for example, when light liquidphase 133 is treated in the PRC removal system, it may be desirable toreturn a portion the raffinate to either the first column, overheaddecanter, or reactor. In some embodiments, the extractant may be furtherdistilled to remove water, which is returned to the one or moreextraction stages. The column bottoms, which contains more methylacetate and methyl iodide than light liquid phase 133, may also berecycled to reactor 105 and/or refluxed to first column 120.

In one embodiment, there is provided a process for producing acetic acidcomprising carbonylating a reactant feed stream comprising methanol,methyl acetate, dimethyl ether, or mixtures thereof in a reactor in thepresence of water, rhodium catalyst, iodide salt and methyl iodide toform a reaction medium in a reactor, introducing a lithium compound intothe reactor, maintaining a concentration of lithium acetate in thereaction medium in an amount from 0.3 to 0.7 wt. %, separating thereaction medium in a flash vessel to form a liquid recycle stream, and avapor product stream comprising acetic acid in an amount from 45 to 75wt. %, methyl iodide in an amount from 24 to less than 36 wt. %, methylacetate in an amount of less than or equal to 9 wt. %, water in anamount of less than or equal to 14 wt. %. The method further comprisesdistilling at least a portion of the vapor product stream in a firstcolumn to obtain an acetic acid product stream comprising acetic acidand hydrogen iodide in an amount of less than or equal to 300 wppm andobtain an overhead stream comprising methyl iodide, water and methylacetate; condensing the overhead stream and phase-separating thecondensing overhead to form a light liquid phase and a heavy liquidphase; and treating a portion of the heavy liquid phase to remove atleast one permanganate-reducing compound selected from the groupconsisting of acetaldehyde, acetone, methyl ethyl ketone, butylaldehyde,crotonaldehyde, 2-ethyl crotonaldehyde, 2-ethyl butyraldehyde, and thealdol condensation products thereof

In some embodiments, the process includes one or more on-line analyzersfor measuring the concentrations of various components in the variousstreams. For example, an on-line analyzer may be used to determine thehydrogen iodide concentration of sidedraw stream 123 by feeding a samplestream, e.g., a sample purge stream, to an on-line analyzer (not shown).

Second Column

Acetic acid removed via sidedraw stream 123 preferably is subjected tofurther purification, such as in a second column 125, also referred toas a drying column. The second column separates sidedraw stream 123 toform an aqueous overhead stream 126 comprising primarily water, andproduct stream 127 comprised primarily of acetic acid. Water from theside stream is concentrated in the aqueous overhead stream and theaqueous overhead comprises greater than or equal to 90% of the water inthe side stream, e.g., greater than or equal to 95%, greater than orequal to 97%, greater than or equal to 99%. Aqueous overhead stream 126may comprise water in an amount from 50 to 90 wt. %, e.g., from 50 to 85wt. %, from 55 to 85 wt. %, from 60 to 80 wt. %, or from 60 to 75 wt. %.In embodiments, aqueous overhead stream may comprise water in an amountof less than or equal to 90 wt. %, e.g., less than or equal to 75 wt. %,less than or equal to 70 wt. %, less than or equal to 65 wt. %. Methylacetate and methyl iodide are also removed from the side stream andconcentrated in the overhead stream. Product stream 127 preferablycomprises or consists essentially of acetic acid and may be withdrawn inthe bottom of second column 125 or a side stream near the bottom. Whenwithdrawn as a side stream near the bottom, the side stream may be aliquid or a vapor stream. In preferred embodiments, product stream 127comprises acetic acid in an amount greater than or equal to 90 wt. %,e.g., greater than or equal to 95 wt. % or greater than or equal to 98wt. %. Product stream 127 may be further processed, e.g., by passingthrough an ion exchange resin, prior to being stored or transported forcommercial use.

Similarly, aqueous overhead stream 126 from second column 125 contains areaction component, such as methyl iodide, methyl acetate, and water,and it is preferable to retain these reaction components within theprocess. Aqueous overhead stream 126 is condensed by a heat exchangerinto stream 138, which is recycled to reactor 105 and/or refluxed secondcolumn 125. An offgas component may be vented via line 137 fromcondensed low-boiling overhead vapor stream 126. Similar to thecondensed low-boiling overhead vapor stream from first column 120,condensed overhead stream 138 may also be separated to form an aqueousphase and an organic phase, and these phases may be recycled or refluxedas needed to maintain the concentrations in the reaction medium.

In one embodiment, the side stream water concentration is controlled tobalance the water in both the first and second columns. When less thanor equal to 14 wt. % water is used in the reaction medium, morepreferably, less than or equal to 4.1 wt. % water there may not besufficient water in the second column to stably operate the column.Although it may be possible to reduce the water concentration in theside stream to less than 1 wt. %, this would result in an imbalance inthe second column, which may cause the recovery of acetic acid to becomemore difficult and therefore result in off-spec product. Further byhaving water in the side stream, the second column can remove that waterin the aqueous overhead. The recycle ratio between the light liquidphase from the first column and the aqueous overhead from the secondcolumn helps to maintain desirable water concentrations in the reactorwhile maintaining stable operations in the first and second distillationcolumns. In one embodiment, the recycle ratio of the mass flow of thelight liquid phase recycled to the reactor to the mass flow of theaqueous overhead to the reactor is less than or equal to 2, e.g., lessthan or equal to 1.8, less than or equal to 1.5, less than or equal to1, less than or equal to 0.7, less than or equal to 0.5, less than orequal to 0.35, less than or equal to 0.25 and/or the recycle ratio ofthe mass flow of the light liquid phase recycled to the reactor to themass flow of the aqueous overhead to the reactor is greater than orequal to 0, e.g., greater than or equal to 0.05, greater than or equalto 0.1, greater than or equal to 0.15, or greater than or equal to 0.2.

Thus, in one embodiment, there is provided a process for producingacetic acid comprising carbonylating a reactant feed stream in thepresence of water, rhodium catalyst, iodide salt and methyl iodide toform a reaction medium in a reactor. The reactant feed stream comprisesmethanol, methyl acetate, dimethyl ether, or mixtures thereof. Theprocess further comprises introducing a lithium compound into thereactor, maintaining a concentration of lithium acetate in the reactionmedium in an amount from 0.3 to 0.7 wt. %; separating the reactionmedium in a flash vessel to form a liquid recycle stream and a vaporproduct stream comprising acetic acid in an amount from 45 to 75 wt. %,methyl iodide in an amount from 24 to less than 36 wt. %, methyl acetatein an amount of less than or equal to 9 wt. %, water in an amount ofless than or equal to 14 wt. %; and distilling at least a portion of thevapor product stream in a first column to obtain an acetic acid productstream comprising acetic acid and hydrogen iodide in an amount of lessthan or equal to 300 wppm, and obtain an overhead stream comprisingmethyl iodide, water and methyl acetate, condensing the first lowboiling overhead vapor stream and biphasically separating the condensedstream to form a heavy liquid phase and a light liquid phase; distillingthe side stream in a second column to obtain a purified acetic acidproduct and a second low boiling overhead vapor stream; condensing thesecond low-boiling overhead vapor stream to obtain an aqueous recyclestream, comprising water in an amount of less than or equal to 90 wt. %;and recycling the second low boiling overhead vapor stream to thereactor, wherein the recycle ratio of the mass flow of the light liquidphase recycled to the reactor to the mass flow of the aqueous recyclestream to the reactor is less than or equal to 2, e.g., from 0 to 2.

To recover residual liquids from the vent stream, in particular lines106, 132, and 137, these lines may be fed to a scrubber that operateswith cooled methanol and/or acetic acid to remove methyl acetate andmethyl iodide. A suitable scrubber is described in U.S. Pat. No.8,318,977, which is incorporated herein by reference in its entirety.

The distillation columns of the present invention may be a conventionaldistillation column, e.g., a plate column, a packed column, and others.Plate columns may include a perforated plate column, bubble-cap column,Kittel tray column, uniflux tray, or a ripple tray column. For a platecolumn, the theoretical number of plates is not particularly limited,and depending on the species of the component to be separate, mayinclude up to 80 plates, e.g., from 2 to 80, from 5 to 60, from 5 to 50,or more preferably from 7 to 35. The distillation column may include acombination of different distillation apparatuses. For example, acombination of bubble-cap column and perforated plate column may be usedas well as a combination of perforated plate column and a packed column.

The distillation temperature and pressure in the distillation system cansuitably be selected depending on the condition such as the species ofthe objective carboxylic acid and the species of the distillationcolumn, or the removal target selected from the lower boiling pointimpurity and the higher boiling point impurity, according to thecomposition of the feed stream. For example, in a case where thepurification of acetic acid is carried out by the distillation column,the inner pressure of the distillation column (usually the pressure ofthe column top) may be from 0.01 to 1 MPa, e.g., from 0.02 to 0.7 MPa,and more preferably, from 0.05 to 0.5 MPa in terms of gauge pressure.Moreover, the distillation temperature for the distillation column,namely the inner temperature of the column at the temperature of thecolumn top, can be controlled by adjusting the inner pressure of thecolumn, and, for example, may be from 20 to 200° C., e.g., from 50 to180° C., and more preferably from 100 to 160° C.

The material of each member or unit associated with the distillationsystem, including the columns, valves, condensers, receivers, pumps,reboilers, and internals, and various lines, each communicating to thedistillation system may be made of suitable materials such as glass,metal, ceramic, or combinations thereof, and is not particularly limitedto a specific one. According to the present invention, the material ofthe foregoing distillation system and various lines are a transitionmetal or a transition-metal-based alloy such as iron alloy, e.g., astainless steel, nickel or nickel alloy, zirconium or zirconium alloythereof, titanium or titanium alloy thereof, or aluminum alloy. Suitableiron-based alloys include those containing iron as a main component,e.g., a stainless steel that also comprises chromium, nickel, molybdenumand others. Suitable nickel-based alloys include those containing nickelas a main component and one or more of chromium, iron, cobalt,molybdenum, tungsten, manganese, and others, e.g., HASTELLOY™ andINCONEL™. Corrosion-resistant metals may be particularly suitable asmaterials for the distillation system and various lines.

Guard Bed

A low total iodide concentration, e.g., up to 5 wppm, e.g., up to 1wppm, in the purified acetic acid product, allows for removal of iodideusing a guard bed. The use of one or more guard beds to remove residualiodide greatly improves the quality of the purified acetic acid product.Carboxylic acid streams, e.g., acetic acid streams, that arecontaminated with halides and/or corrosion metals may be contacted withthe ion exchange resin composition under a wide range of operatingconditions. Preferably, the ion exchange resin composition is providedin a guard bed. The use of guard beds to purify contaminated carboxylicacid streams is well documented in the art, for example, U.S. Pat. Nos.4,615,806; 5,653,853; 5,731,252; and 6,225,498, which are herebyincorporated by reference in their entireties. Generally, a contaminatedliquid carboxylic acid stream is contacted with the ion exchange resincomposition, which is preferably disposed in the guard bed. The halidecontaminants, e.g., iodide contaminants, react with the metal to formmetal iodides. In some embodiments, hydrocarbon moieties, e.g., methylgroups, that may be associated with the iodide may esterify thecarboxylic acid. For example, in the case of acetic acid contaminatedwith methyl iodide, methyl acetate would be produced as a byproduct ofthe iodide removal. The formation of this esterification producttypically does not have a deleterious effect on the treated carboxylicacid stream.

In one embodiment, the ion exchange resin is a metal-exchanged ionexchange resin and may comprise at least one metal selected from thegroup consisting of silver, mercury, palladium and rhodium. In oneembodiment, at least 1% of the strong acid exchange sites of saidmetal-exchanged resin are occupied by silver. In another embodiment, atleast 1% of the strong acid exchange sites of said metal-exchanged resinare occupied by mercury. The process may further comprise treating thepurified acetic acid product with a cationic exchange resin to recoverany silver, mercury, palladium or rhodium.

The pressure during the contacting step is limited primarily by thephysical strength of the resin. In one embodiment, the contacting isconducted at pressures ranging from 0.1 MPa to 1 MPa, e.g., from 0.1 MPato 0.8 MPa or from 0.1 MPa to 0.5 MPa. For convenience, however, bothpressure and temperature preferably may be established so that thecontaminated carboxylic acid stream is processed as a liquid. Thus, forexample, when operating at atmospheric pressure, which is generallypreferred based on economic considerations, the temperature may rangefrom 17° C. (the freezing point of acetic acid) to 118° C. (the boilingpoint of acetic acid). It is within the purview of those skilled in theart to determine analogous ranges for product streams comprising othercarboxylic acid compounds. The temperature of the contacting steppreferably is kept relatively low to minimize resin degradation. In oneembodiment, the contacting is conducted at a temperature ranging from25° C. to 120° C., e.g., from 25° C. to 100° C. or from 50° C. to 100°C. Some cationic macroreticular resins typically begin degrading (viathe mechanism of acid-catalyzed aromatic desulfonation) at temperaturesof 150° C. Carboxylic acids having up to 5 carbon atoms, e.g., up to 3carbon atoms, remain liquid at these temperatures. Thus, the temperatureduring the contacting should be maintained below the degradationtemperature of the resin utilized. In some embodiments, the operatingtemperature is kept below temperature limit of the resin, consistentwith liquid phase operation and the desired kinetics for halide removal.

The configuration of the guard bed within an acetic acid purificationtrain may vary widely. For example, the guard bed may be configuredafter a drying column. Additionally or alternatively, the guard be maybe configured after a heavy ends removal column or finishing column.Preferably, the guard bed is configured in a position wherein thetemperature acetic acid product stream is low, e.g., less than or equalto 120° C. or less than or equal to 100° C. Aside from the advantagesdiscussed above, lower temperature operation provides for less corrosionas compared to higher temperature operation. Lower temperature operationprovides for less formation of corrosion metal contaminants, which, asdiscussed above, may decrease overall resin life. Also, because loweroperating temperatures result in less corrosion, vessels advantageouslyneed not be made from expensive corrosion-resistant metals, and lowergrade metals, e.g., standard stainless steel, may be used.

In one embodiment, the flow rate through the guard bed ranges from 0.1bed volumes per hour (“BV/hr”) to 50 BV/hr, e.g., 1 BV/hr to 20 BV/hr orfrom 6 BV/hr to 10 BV/hr. A bed volume of organic medium is a volume ofthe medium equal to the volume occupied by the resin bed. A flow rate of1 BV/hr means that a quantity of organic liquid equal to the volumeoccupied by the resin bed passes through the resin bed in a one hourtime period.

To avoid exhausting the resin with a purified acetic acid product thatis high in total iodide concentration, in one embodiment the purifiedacetic acid product in bottoms stream 127 is contacted with a guard bedwhen total iodide concentration of the purified acetic acid product isup to 5 wppm, e.g., preferably up to 1 wppm. In one exemplaryembodiment, the total iodide concentration of the purified acetic acidproduct may be from 0.01 wppm to 5 wppm, e.g., from 0.01 wppm to 1 wppm.Concentrations of iodide above 5 wppm may require re-processing theoff-spec acetic acid. Total iodide concentration includes iodide fromboth organic, C₁ to C₁₄ alkyl iodides, and inorganic sources, such ashydrogen iodide. A purified acetic acid composition is obtained as aresult of the guard bed treatment. The purified acetic acid composition,in one embodiment, comprises less than or equal to 100 wppb iodides,e.g., less than or equal to 90 wppb, less than or equal to 50 wppb, orless than or equal to 25 wppb. In one embodiment, the purified aceticacid composition comprises less than or equal to 1000 wppb corrosionmetals, e.g., less than or equal to 750 wppb, less than or equal to 500wppb, or less than or equal to 250 wppb. In terms of ranges, thepurified acetic acid composition may comprise from 0 to 100 wppbiodides, e.g., from 1 to 50 wppb; and/or from 0 to 1000 wppb corrosionmetals, e.g., from 1 to 500 wppb. In other embodiments, the guard bedremoves at least 25 wt. % of the iodides from the crude acetic acidproduct, e.g., at least 50 wt. % or at least 75 wt. %. In oneembodiment, the guard bed removes at least 25 wt. % of the corrosionmetals from the crude acetic acid product, e.g., at least 50 wt. % or atleast 75 wt. %.

In another embodiment, the product stream may be contacted with cationicexchanger to remove lithium compounds. The cationic exchanger in theacid form comprises a resin of acid-form strong acid cation exchangemacroreticular, macroporous or mesoporous resins. Without being bound bytheory feeding a product stream to an ion-exchange comprising lithiumcompounds in an amount of greater than or equal to 10 wppm results indisplacement of metals in the treated product. Advantageously, this maybe overcome by using an cationic exchanger upstream of the ion-exchangeresin. After contacting with the cationic exchanger, the product streammay have a lithium ion concentration of less than 50 weight part perbillion (wppb), e.g., less than 10 wppb, or less than 5 wppb.

Although the product stream may be contacted with an ion-exchange resinto remove iodides, it is preferred not to flash the product stream orcontact with product stream with an adsorption system that containsactivated carbon. Flashing the product stream is not efficient becausethere is not a sufficient pressure drop to recover greater than 50% ofthe acetic acid from the product stream. Thus, in one embodiment, anon-flashed portion of the product stream is fed to the ion-exchange bedto remove iodides.

As is evident from the figures and text presented above, a variety ofembodiments are contemplated.

E1. A process for producing acetic acid, comprising:

carbonylating a reactant feed stream comprising methanol, methylacetate, dimethyl ether, or mixtures thereof in a reactor in thepresence of water, rhodium catalyst, iodide salt and methyl iodide toform a reaction medium in a reactor;

introducing a lithium compound into the reactor;

maintaining a concentration of lithium acetate in the reaction medium inan amount from 0.3 to 0.7 wt. %;

separating the reaction medium in a flash vessel to form a liquidrecycle stream and a vapor product stream comprising acetic acid in anamount from 45 to 75 wt. %, methyl iodide in an amount from 24 to lessthan 36 wt. %, methyl acetate in an amount of less than or equal to 9wt. %, and water in an amount of less than or equal to 14 wt. %; and

distilling at least a portion of the vapor product stream in a firstcolumn to obtain an acetic acid product stream comprising acetic acidand hydrogen iodide in an amount of less than or equal to 300 wppm andan overhead stream comprising methyl iodide, water and methyl acetate.

E2. The process of embodiment E1, wherein the vapor product streamfurther comprises acetaldehyde in an amount from 0.005 to 1 wt. %.

E3. The process of any one embodiments E1-E2, wherein the vapor productstream further comprises acetaldehyde in an amount from 0.01 to 0.8 wt.%.

E4. The process of any one embodiments E1-E3, wherein the vapor productstream comprises acetic acid in an amount from 55 to 75 wt. %, methyliodide in an amount from 24 to 35 wt. %, methyl acetate in an amountfrom 0.5 to 8 wt. %, water in an amount from 0.5 to 14 wt. %, andfurther comprises hydrogen iodide in an amount less than or equal to 0.5wt. %.E5. The process of any one embodiments E1-E4, wherein the vapor productstream comprises acetaldehyde in an amount from 0.01 to 0.7 wt. %.E6. The process of embodiment E5, wherein the vapor product streamcomprises acetic acid in an amount from 60 to 70 wt. %, methyl iodide inan amount from 25 to 35 wt. %, methyl acetate in an amount from 0.5 to6.5 wt. %, water in an amount from 1 to 8 wt. %, acetaldehyde in anamount from 0.01 to 0.7 wt. %, and hydrogen iodide in an amount lessthan or equal to 0.1 wt. %.E7. The process of any one embodiments E1-E6, wherein the vapor productstream comprises hydrogen iodide in an amount less than or equal to 1wt. %.E8. The process of any one embodiments E1-E7, further comprising ventingfrom the reactor a gaseous stream that comprises hydrogen iodide in anamount of less than or equal to 1 wt. %.E9. The process of embodiment E8, wherein the gaseous stream compriseshydrogen iodide in an amount from 0.001 to 1 wt. %.E10. The process of any one embodiments E1-E9, wherein the overheadstream is phase separated to form a light liquid phase and a heavyliquid phase.E11. The process of embodiment E10, wherein the light liquid phasecomprises acetic acid in an amount from 1 to 40 wt. %, methyl iodide inan amount of less than or equal to 10 wt. %, methyl acetate in an amountfrom 1 to 50 wt. %, water in an amount from 40 to 80 wt. %, acetaldehydein an amount of less than or equal to 5 wt. %, and hydrogen iodide in anamount of less than or equal to 1 wt. %.E12. The process of embodiment E10, wherein the light liquid phasecomprises acetic acid in an amount from 5 to 15 wt. %, methyl iodide inan amount of less than or equal to 3 wt. %, methyl acetate in an amountfrom 1 to 15 wt. %, water in an amount from 70 to 75 wt. %, acetaldehydein an amount from 0.1 to 0.7 wt. %, and hydrogen iodide in an amountfrom 0.001 to 0.5 wt. %.E13. The process of embodiment E10, wherein a portion of the heavyliquid phase is treated to remove at least one permanganate reducingcompound selected from the group consisting of acetaldehyde, acetone,methyl ethyl ketone, butylaldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, 2-ethyl butyraldehyde, and the aldol condensationproducts thereof.E14. The process of embodiment E10, wherein a portion of the lightliquid phase is returned to the reactor.E15. The process of any one embodiments E1-E14, wherein the liquidrecycle stream comprises a rhodium catalyst in an amount from 0.01 to0.5 wt. %, lithium iodide in an amount from 5 to 20 wt. %, corrosionmetals in an amount from 10 to 2500 wppm, acetic acid in an amount from60 to 90 wt. %, methyl iodide in an amount from 0.5 to 5 wt. %, methylacetate in an amount from 0.1 to 5 wt. %, water in an amount from 0.1 to8 wt. %, acetaldehyde in an amount from 0.0001 to 1 wt. %, and hydrogeniodide in an amount from 0.0001 to 0.5 wt. %.E16. The process of any one embodiments E1-E15, wherein the lithiumcompound is selected from the group consisting of lithium acetates,lithium carboxylates, lithium carbonates, lithium hydroxides, andmixtures thereof.E17. The process of any one embodiments E1-E16, further comprisingmaintaining a concentration of hydrogen iodide in the reaction medium inan amount from 0.1 to 1.3 wt. %.E18. The process of any one embodiments E1-E17, wherein thecarbonylation is conducted while maintaining a carbon monoxide partialpressure from 2 to 30 atm and a hydrogen partial pressure in the reactorthat is less than or equal to 0.04 atm.E19. The process of any one embodiments E1-E18, wherein theconcentration of methyl acetate in the reaction medium is greater thanthe concentration of the lithium acetate in the reaction medium.E20. The process of any one embodiments E1-E19, wherein the acetic acidproduct stream comprises each of the methyl iodide and the methylacetate in an amount within the range of ±0.9 wt. % of the waterconcentration in the side stream.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference. In addition, it should beunderstood that aspects of the invention and portions of variousembodiments and various features recited below and/or in the appendedclaims may be combined or interchanged either in whole or in part. Inthe foregoing descriptions of the various embodiments, those embodimentswhich refer to another embodiment may be appropriately combined withother embodiments as will be appreciated by one of skill in the art.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention.

EXAMPLES

The present invention will be better understood in view of the followingnon-limiting examples.

Example 1

The reactor, containing approximately 900 ppm by weight of rhodium inthe form of a rhodium carbonyl iodide compound and approximately 9 wt. %methyl iodide as well as lithium iodide, water, and methyl acetate, wasfed with methanol, carbon monoxide and hydrogen to maintain a hydrogenpartial pressure of at least about 0.27 atm (i.e. at least about 4 psi).The water concentration was less than 4 wt. %. The reactor maintained atemperature between about 190° C. and 200° C. and operated at a pressureabove 28 atm (i.e. above 400 psig). By means of a level control sensingthe liquid level within the reactor, liquid reaction medium wascontinuously withdrawn and fed to the a single-tray flash vesseloperating at approximately 150° C. and approximately 3.2 atm (i.e. 32.3psig). Carbon monoxide recovered from the reactor vent was spraged intothe withdrawn reaction medium prior to entering the flash vessel. About28% of reaction medium exits the as the vapor product stream and theremaining amount is returned as a liquid to the reactor. The vaporproduct stream was withdrawn at a temperature of approximately 50° C.The vapor product stream composition was as follows: 66.35 wt. % aceticacid, 25.01 wt. % methyl iodide; 5.97 wt. % methyl acetate, 1.53 wt. %water, 0.1 wt. % acetaldehyde, and less than 1 wt. % hydrogen iodide.

The vapor product stream was fed to a light ends column to obtain anoverhead and a sidedraw stream. A typical example of the HIconcentration in sidedraw stream was determined by titrating asufficient amount of sidedraw stream sample with 0.01 M lithium acetatesolution in 50 ml acetone. A pH electrode was used with Metrohm 716 DMSTitrino to determine the end point at Dynamic Equivalence-pointTitration mode. HI concentration in wt. % was calculated based on theconsumption of lithium acetate titrant as depicted in followingequation.

${{HI}\mspace{14mu}{{wt}.\mspace{14mu}\%}} = \frac{\left( {{ml}\mspace{14mu}{of}\mspace{14mu}{LiOAc}} \right)\left( {0.01M} \right)\left( {128\mspace{14mu} g\text{/}{mole}} \right) \times 100}{\left( {g\mspace{14mu}{sample}} \right)\left( {1000\mspace{14mu}{ml}\text{/}L} \right)}$

A sample sidedraw stream composition having about 1.9 wt. % water, about2.8 wt. % methyl iodide, and about 2.5 wt. % methyl acetate, was testedusing this HI titration method. The HI concentrations varied from 50wppm to 300 wppm, when there was a recycle of the light liquid phasefrom the overhead light ends to the reaction system. Thus, maintainingthe methyl iodide concentration in the vapor product stream contributedto controlling the HI concentrations in the light ends column.

Example 2

The reaction of Example 1 was repeated except the methyl iodide in thereaction medium was approximately 12 wt. % and the pressure of the flashvessel was slightly lower, approximately 3.1 atm (i.e. 30.8 psig). About31% of reaction medium exited as the vapor product stream and theremaining amount was returned as a liquid to the reactor. The vaporproduct stream composition was as follows: 61.97 wt. % acetic acid,30.34 wt. % methyl iodide; 5.05 wt. % methyl acetate, 1.54 wt. % water,0.09 wt. % acetaldehyde, and less than 1 wt. % hydrogen iodide.

A portion of the light liquid phase from the light ends overhead wasrecycled to the reaction system. The sidedraw stream contained 1.5 wt. %water, 3.6 wt. % methyl acetate, 2.1 wt. % methyl iodide, and less than25 wppm HI, and the balance comprised acetic acid. HI concentrationswere too low to measure directly with titration. The presence of othercations in the sidedraw made directly measuring HI difficult. Themeasure of total inorganic iodide, i.e., total possible maximized HI,was done directly. Other inorganic iodides may include lithium iodide,as well as corrosion metal iodide. Again, maintaining the methyl iodideconcentration in the vapor product stream beneficially contributed tocontrolling the HI concentration in the light ends column and ultimatelyin the sidedraw stream.

What is claimed is:
 1. A process for producing acetic acid, comprising:carbonylating a reactant feed stream comprising methanol, methylacetate, dimethyl ether, or mixtures thereof in a reactor in thepresence of water, rhodium catalyst, iodide salt and methyl iodide toform a reaction medium in a reactor; introducing a lithium compound intothe reactor wherein the lithium compound is selected from the groupconsisting of lithium acetate, lithium carbonate, lithium hydroxide, andmixtures thereof; maintaining the concentration of lithium acetate inthe reaction medium in an amount from 0.3 to 0.7 wt. %; separating thereaction medium in a flash vessel to form a liquid recycle stream and avapor product stream comprising acetic acid in an amount from 45 to 75wt. %, methyl iodide in an amount from 24 to less than 36 wt. %, methylacetate in an amount of less than or equal to 9 wt. %, and water in anamount of less than or equal to 14 wt. %; and distilling at least aportion of the vapor product stream in a first column to obtain anacetic acid product stream comprising acetic acid and hydrogen iodide inan amount of less than or equal to 300 wppm and an overhead streamcomprising methyl iodide, water and methyl acetate.
 2. The process ofclaim 1, wherein the vapor product stream further comprises acetaldehydein an amount from 0.005 to 1 wt. %.
 3. The process of claim 1, whereinthe vapor product stream further comprises acetaldehyde in an amountfrom 0.01 to 0.8 wt. %.
 4. The process of claim 1, wherein the vaporproduct stream comprises acetic acid in an amount from 55 to 75 wt. %,methyl iodide in an amount from 24 to 35 wt. %, methyl acetate in anamount from 0.5 to 8 wt. %, water in an amount from 0.5 to 14 wt. %, andfurther comprises hydrogen iodide in an amount less than or equal to 0.5wt. %.
 5. The process of claim 1, wherein the vapor product streamcomprises acetaldehyde in an amount from 0.01 to 0.7 wt. %.
 6. Theprocess of claim 5, wherein the vapor product stream comprises aceticacid in an amount from 60 to 70 wt. %, methyl iodide in an amount from25 to 35 wt. %, methyl acetate in an amount from 0.5 to 6.5 wt. %, waterin an amount from 1 to 8 wt. %, acetaldehyde in an amount from 0.01 to0.7 wt. %, and hydrogen iodide in an amount less than or equal to 0.1wt. %.
 7. The process of claim 1, wherein the vapor product streamcomprises hydrogen iodide in an amount less than or equal to 1 wt. %. 8.The process of claim 1, further comprising venting from the reactor agaseous stream that comprises hydrogen iodide in an amount of less thanor equal to 1 wt. %.
 9. The process of claim 8, wherein the gaseousstream comprises hydrogen iodide in an amount from 0.001 to 1 wt. %. 10.The process of claim 1, wherein the overhead stream is phase separatedto form a light liquid phase and a heavy liquid phase.
 11. The processof claim 10, wherein the light liquid phase comprises acetic acid in anamount from 1 to 40 wt. %, methyl iodide in an amount of less than orequal to 10 wt. %, methyl acetate in an amount from 1 to 50 wt. %, waterin an amount from 40 to 80 wt. %, acetaldehyde in an amount of less thanor equal to 5 wt. %, and hydrogen iodide in an amount of less than orequal to 1 wt. %.
 12. The process of claim 10, wherein the light liquidphase comprises acetic acid in an amount from 5 to 15 wt. %, methyliodide in an amount of less than or equal to 3 wt. %, methyl acetate inan amount from 1 to 15 wt. %, water in an amount from 70 to 75 wt. %,acetaldehyde in an amount from 0.1 to 0.7 wt. %, and hydrogen iodide inan amount from 0.001 to 0.5 wt. %.
 13. The process of claim 10, whereina portion of the heavy liquid phase is treated to remove at least onepermanganate reducing compound selected from the group consisting ofacetaldehyde, acetone, methyl ethyl ketone, butylaldehyde,crotonaldehyde, 2-ethyl crotonaldehyde, 2-ethyl butyraldehyde, and thealdol condensation products thereof.
 14. The process of claim 10,wherein a portion of the light liquid phase is returned to the reactor.15. The process of claim 1, wherein the liquid recycle stream comprisesa rhodium catalyst in an amount from 0.01 to 0.5 wt. %, lithium iodidein an amount from 5 to 20 wt. %, corrosion metals in an amount from 10to 2500 wppm, acetic acid in an amount from 60 to 90 wt. %, methyliodide in an amount from 0.5 to 5 wt. %, methyl acetate in an amountfrom 0.1 to 5 wt. %, water in an amount from 0.1 to 8 wt. %,acetaldehyde in an amount from 0.0001 to 1 wt. %, and hydrogen iodide inan amount from 0.0001 to 0.5 wt. %.
 16. The process of claim 1, furthercomprising maintaining a concentration of hydrogen iodide in thereaction medium in an amount from 0.1 to 1.3 wt. %.
 17. The process ofclaim 1, wherein the carbonylation is conducted while maintaining acarbon monoxide partial pressure from 2 to 30 atm and a hydrogen partialpressure in the reactor that is less than or equal to 0.04 atm.
 18. Theprocess of claim 1, wherein the concentration of methyl acetate in thereaction medium is greater than the concentration of the lithium acetatein the reaction medium.